Some old school options for a AA or AAA powered taillight:
You might also look for a AA powered flashlight with a red LED. There are ways to mount those backwards.
The flashlight probably would give you good throw but not much light left and right. If you want to cover all bases with AA and AAA lights you could mount the flashlight center and 2 Superflash Turbos directed 10 degrees left and right from each seat stay.
Most current tail lights, and perhaps all the brightest models, are now powered by rechargables other than AAs or AAAs.