Thanks. turns out the PDW Radboot 1000 comes with a rack light for free. Its battery operated which isn't my preference, but I like that it has a built in reflector and a lot of people on this forum seem to like it. The price is right so I may order one to try it. Worse case it can go on a backup bike or my wife's bike. Bonus is rack mount sounds like it would work with other PDW as well as Planet Bike lights.
The other light I saw as a good option is the Cygolight Hotshot. It is highly rated and you can bet a rack mount for $3-$8.