I bought a taillight and strobe that had only seatpost mounts. However, I wanted to mount them on the back of a rear rack (the racktrunk bag hides the seatpost). Thought I'd share my solution FWIW.
Slice off most of an approx-seatpost-size PVC elbow to leave a tab. Bandsaw or hacksaw, then sanding. In my case, my ancient rack has a horizontal flat area at the back. Drill holes and bolt it where desired. A "vertical" tab can be made using a straight piece of PVC. The tab is pretty sturdy BTW.