I've pondered on this as well. I like the taillight idea and especially the marker light idea but it looks pretty cumbersome trying to get the pump nozzle in there. I've never seen it done in person so maybe I am wrong. At this point I am leaning towards grafting in a fuel door. I think everyone I have seen is a square door, I am considering using an 88-98 style truck filler which is round. I think it might go better with the body lines, not sure though...
Heres a random pic for reference.
