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I work as a delivery man on a bicycle, so I often ride my bike after dark with a black pizza box attached. My rear light attaches to my seatpost, and is completely useless when the pizza box is there.
Are there any lights designed to attach to the pizza box, which has not many obvious features except for a clasp, or is there some good way of mounting a standard light to the box itself?
One possibility is to mount the tail light on your bike's seat stays. As an example here are a couple pictures of my preferred tail light (the cygolite hotshot) mounted to the seat stay of a cross bike:
There are a bunch of options out there for this sort of mounting, from lights specifically designed to be mounted on a seat stay to ones that come with seat stay mounts in addition to the normal seat post mount.
A few things you want to keep in mind when choosing/using a seat post mounted tail light:
Another possibility is a rack mounted light. Again like the seat stay mounted tail lights, there are lights specifically designed for rack mounting as well as mounts for that allow other tail lights to be used with racks. In the example of the cygolite tail light I use, there is an optional rack mount available, although I was actually able to adapt an old rear reflector mount to serve in this capacity.
As Stephan Matthiesen notes in the comments, if your pizza box is mounted on a standard bicycle rack, there are rear lights that attach to the rack and should be visible even with the box on the rack. Or you could do as Glenn Stevens suggests in his answer, and get a light that attaches to the seat stays of your bike.
If you really want to attach a light to the pizza box itself (and don't just want to glue or bolt it in place — I assume the pizza place might complain about that), you might want to consider a red clip-on light, or perhaps a little magnetic light like these.* These are meant to attach to clothing or to saddle bags, but there's a good chance that either or both of those might also attach to the clasp on the pizza box, if it's metallic and/or has some kind of a handle you could clip the light to.
Or, if you like the idea of a magnetic light, but the ones sold for joggers feel too small, you could probably make one yourself by supergluing a couple of neodymium magnets to a basic cheap tail light (possibly after hacksawing any extra bits out of the way).
Those products also suggest yet another alternative solution: instead of attaching the light to either the box or the bike, attach it to yourself. Besides the clothing-mounted options mentioned above, there are also rear lights that attach to your helmet.* I assume your pizza box isn't so big that it would obscure that.
Also, remember that for safety, reflectors are just as important as lights, if not more so. You should be able to find some reflective safety tape or stickers that you can attach to either your bike or (if the pizza shop permits it) to the pizza box. Also consider wearing high-visibility clothing, or a reflective vest over your normal clothes. Wheel and/or pedal reflectors are also highly advisable for urban night riding — they might look "dorky" to some, but their distinctive motion instantly screams "bicycle!" to any driver whose headlights you may find yourself in.
*) The linked products are random search results from Amazon for illustrative purposes. You local bike store, or even a well stocked supermarket, will likely have something similar.
Reflective tape is the way to go if you have a big surface area. I'm not going to link to a product because it is easy enough to find.
