There are few rechargeable lights with beam shaping and cut-off that may be mounted upside down. Cat-Eye has two in its G-Volt line that come with GoPro adapters. Lezyne also has a light for hanging mount.
GoPro mounts below Garmin (or other head unit) mounts are fairly common.
Below is a picture of a GVolt 70 mounted to a Sigma mount. No adapters or small ...
I’ve built this mount out of some aluminium sheet I had lying around:
It uses the two existing holes in the bottom of the Garmin mount for two small screws. The heads of the screws are inside of the mount (they barely fit in the space below the topside mounting mechanism). The blue stuff is threadlocker.
Sometimes it is impossible to find exactly the combination of features you need in a small part.
Presumably you need a "quarter turn lock" receiver on top to mount with the underside of your garmin mount, and then below that some kind of round "bar" for the light to strap onto ?
But instead of the gopro mount in this picture, you want a ...