I needed an adaptor plate to mount my tail light to the license-plate-region of my Topeak bike rack. This style of tail light seems pretty common, so I hope this helps someone else out.

Update

After using this for a couple days on the trail, my light did pop out once after a jostling bump. I've temporarily fixed this with a hack -- wrapping a rubber band or hair elastic around it to keep it held down. When I get some more time, I'll be adding a mechanism to help keep it clipped or locked in place. The open scad file even has a lot of the code necessary to build in a clip in comments, but some modifications, iteration, and trial-and-error will be necessary.