This is a rack tail light I found after doing some research:
https://www.serfas.com/products/view...ights%7Cpage:6

It's on sale at biketiresdirect for $15 plus $5 delivery. Runs on two AAs (included) which can be recharged through USB port. The rack bracket is $5 extra if you wish to make it removable. I mounted it permanently by the means of a bolt through the body and into the rack. 35 lumen and wide throw lens make it visible even during a not so sunny day. Quite satisfied....