I've fitted a Garmin Varia rear light (not the RTL510 though) to a rear rack on my Electra Townie cruiser using a couple of parts. This may help someone who already has a rear rack with reflector mount holes and wants to fit a Garmin Varia rear light onto the rack instead of the seatpost. It uses one of the reflector mount holes on a rear rack.
First I ordered a K-Edge Gravity Top Cap Mount that usually replaces your headset top cap:
https://k-edge.com/shop/computer-mou...top-cap-mount/
I also needed an M6 self-locking nut (and a washer for it):
https://www.amazon.co.uk/dp/B00DY2CTXC/
Here's all the bits shown together with the K-Edge headset cap disassembled:
Attach the cap to the reflector mount on the rack using the bolt supplied with the cap and the self-locking nut:
Refit the Garmin mount insert into the cap:
Here's how it looks:
And here's the rack mounted on the bike showing the Garmin Varia rear light:
