I have several of the free Harbor Freight blue puck lights - very bright.
Any ideas how to attach to old 1st generation Specialized Hard Rock mountain bike ?
I have one on now with reusable straps right but that's not going to work, will come off even on road riding.
I think I'm going to have to drill the lights and mount to a bracket.
Edit: I had not considered the point about these lights being flood lights, and I agree that they're not adequate as main front lights.
I was tempted to delete this answer, but I think the direct answer to question on "how to mount odd shaped object to bike?" is valid and there are valid use cases for floodlights in bikes, in my case, for night MTB riding. I'm primarily an MTB cyclist and where I live there are no legal regulations on bike lights. In regard to blinding other road/path users, I'm aware of the type of lights I use and I turn them off or aim them away from other people's faces.
Flood light in the handlebar are recommended, for example in rocky and slow climbs, where you swing the steering repeatedly to keep balance. The flood light pointed down and is used to illuminate the nearest couple of meters ahead, starting from the front wheel contact patch onward. My setup includes a helmet mounted "spotlight" that illuminates further away ant that I can aim where I'm looking at while the handlebar may be pointing in another direction.
Yet another point for MTB use is that you need some upwards lighting in case of a low branch or such, and also, you're not on level ground, so a spotlight may point too high in one part of the trail and too low a few metres after.
The low power of such units actually makes them less likely to blind or dazzle other path users, specially pointed not straight ahead. In complete darkness, after eyes have adapted, even the limited output becomes use-able and does not take the eyes out of "night vision mode" that much.
Also, Indeed flood lights are of little use for high speed riding, neither road or MTB. They do not illuminate far enough to react to any danger.
End of edit
I once made a cell phone mount with some PVC scraps and a hose clamp. The PVC was a bit thicker than 1 mm or about 1/16. I pre cut a shape from a flattened portion of PVC pipe(Imagine a rectangle, about the same size of the phone, to which you add trapezoidal flaps on each side) , then softened it on boiling water and grabbing it with pliers and with kitchen gloves, I wrapped it around the cell phone, making the flaps into small hooks that grabbed the phone. The phone was in a silicon protective sleeve.
Then I made two slits in the flat part of the PVC slat and threaded a hose clamp trough the slits and then tightened it around the handlebar. A strip from old inner tube, rubber hose scraps, foam tape or certain types of handlebar tape can be used line the handlebar to prevent scratches. It also increases friction so you don't need to tighten the clamp too much for the assembly to be stable. I used foamy handlebar tape that had adhesive backing that made it easier to install the clamp.
You could perform a similar approach for the lights, but the hot PVC may deform the light's casing, so you may take a scrap of wood and roughly shape it to similar size of the lights and wrap the PVC around that.
In my case the bracket was permanently attached to the bike, the tabs flexibility allowed me to take the phone in and out with little difficulty, yet it was secure enough to jump down from a curb or ride over a speed bump without letting the phone fall. I used it for urban navigation for a time when I was on delivery.
You can make this bracket out of sheet metal, specially if you can leave the lights permanently on the bracket. You can also make only 3 tabs and closing the fourth side with a protruding bolt, so you can remove the lights when needed.
Instead of hose clamps, you can also use some pipe clamps normally used to secure electrical conduit to walls. They are plenty strong for the small weight of a light and are already shaped to hold on to cylindrical shapes.
You can also simply make a flat backing for the light (Out of wood or metal), with the slits for the clamp and adhere the light to that with double sided mounting tape or epoxy. (if you don't obstruct any functionality with that.)
A third option is a wooden strip ( slightly longer than the lights. Adhere a small wood block as a stopper on either side to prevent the light from sliding off. If made from metal, simply cut a strip longer than the light and bend a little portion on each side to shape it like a square bracket "[". Then use zip ties to attach the light to the strip. You can attach the wooden or metal strip to the bike with more zip ties, hose clamps or pipe clamps.
You mention in the comments that you’re almost entirely riding on the road. The pictured light is (based on having used them for other purposes myself) essentially useless for illuminating things so that you can see, and is dangerously bright for illuminating your bike so you can be seen depending on how you mount it.
A light for illuminating the road ahead of you needs a pretty specific geometry of the beam to be effective. It has to have enough ‘throw’ (effective max range) on the ground to clearly see stuff far enough ahead that you can safely avoid hazards like potholes, and it has to have a wide enough spread at that distance that you can also see that it’s safe to go around whatever hazard you are avoiding. The spot lighting on your light is far too narrow for safe usage, and the flood lighting mode does not have sufficient throw to be useful.
Additionally, it’s far too bright for a light without a proper vertical cutoff. Any oncoming traffic is likely to be dazzled (as in, their vision will be temporarily impaired) by it, which is seriously dangerous for both them and you. Good lights have a vertical cutoff just like a car’s headlights, so that very little light is projected upwards, avoiding this issue (and also making the light more efficient, because any light going upwards is generally wasted). Some countries (for example, Germany) actually legally require that any forward vehicle light (including bicycle lights) be designed this way, and noncompliance in such places is very expensive.
If you need a front light, it’s worth shelling out for a good one because it will both make you safer, and make things safer for other road users. If your country does not mandate a vertical cutoff, you can usually find lights with them by adding ‘stvzo’ to your search terms (this is the abbreviation for the name of the German traffic code that mandates vertical cutoffs, and seems to be what most of the industry has standardized on as the keyword for indicating lights with vertical cutoffs).
Now, this does not mean this type of light is completely useless for a cyclist.
They are great if you’ve got to stop and deal with a mechanical issue on the side of the road (far better than trying to use a cellphone camera for this), and they are also useful for illuminating thin clothing or bags from the inside (which does wonders for visibility).
Additionally, provided it’s not illegal where you live, you could use the wide beam for somewhat bright underglow lighting, which can also help significantly with making you (safely) visible to other road users.
Realistically, this tool is for lighting up your worksite and is unsuitable for use as vehicle illumination on the road.
Why?
The focused spotlight on the end is a tight beam, and is too narrow to effectively light your way.
Meanwhile the broad strip on the side is the opposite, with a wide flood of light in all directions - it will spray light upward as much as downward, and won't have much "throw".
A good front light will have a D shaped pattern of light such that it doesn't rise above the horizontal and blind/dazzle other road users.
As a white light, it cannot be used on the rear at any time.
The best use for a light with beam-forms like this is in your bag, for roadside repairs or similar.
Or salvage the battery/charger-circuit and use it to provide long-duration power to an existing well-designed bike light.
Another use might be inside your clothing. I have a red lamp, and carrying it inside a white canvas backpack gives an entirely illuminated pack. Works well in the black.
Here are some rough ideas.
Your solution will be based on your skill set and tools.
Here is the back of your light.
There are three screws in the back of the light that hold the back on.
If make a handlebar mount bracket - or scavenge one GoPro bracket or other - this would make a good 3D print project - and use three longer screws to hold the scavenged/ made bracket in a sandwich and leverage the existing holes. The bracket may be as simple as a small square of wood or metal or plastic and a pipe clamp or zip ties or velcro.
Rough picture
Another option - instead of using screws to mount a bracket - would be to use epoxy to hold the bracket to the back.
