This is a Mount for a Supernova Lamp on a brompton. It fits between mudguard and brake.

v8: the lamp is not hold in position when its shake. maybe you can clue it.
v16 Space for a ring with tooth (Zahnscheibe in germany) that holds the lamp pretty good in place.

Prusa

I3 MK3S

25

I printed it on the side with supports to got more Strength on the holes and it works.
Use 4 shells/walls to get it stronger.