I just realized that Fenix sells an optional red filter that you can use rather than the standard clear lens cap. They've got it hidden in a strange place on their web site, under Holsters/Boxes.

https://www.fenix-store.com/product_...6d207b00e0a254

Anyway, it seems like you could easily use a Fenix for tail-light using one of these filters. Should be simple to mount the light to a seatpost or helmet using the TwoFish lockblocks or new bike mount. The only potential issue is that Fenix lights -- my model anyway, the L2D Premium -- don't seem to have very good options for a blinking mode. It will blink rapidly in turbo model, but that cuts the run time. In regular mode, the SOS flash is too slow and irregular. However, if you kept the light on a steady mode, it seems like it would be comparable to a DiNotte tail-light for about 1/2 the price.