This is what you want if you need to mount a Planet Bike superflash or other tail light with a similar mounting system to your bicycle. PlanetBike makes great little tail lights and they provide a seat post mount, but if you've got a rack and need a center-mount tail light, that's an extra charge.

This uses a standard 6mm hex-head screw, which is what pretty much everything attaches to a bike with. The head is counter-sunk so there is clearance for the light. Totally sourced the light interface bits from thingiverse.com/thing:8947 so big thanks to gpvillamil for all that fussy work.