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 parts were needed. Both were compatible out of the box. For beam characteristics and review (in German) see [Boissier-Mohr's webpage][2]

[![GVOLT 70 light mounted hanging on GoPro mount][1]][1]


  [1]: https://i.sstatic.net/vo0tw.jpg
 [2]: https://fahrradbeleuchtung-info.de/testbericht-cateye-gvolt70