This thread from another board should give you some good ideas. Make sure you look at the links to other threads on the board, since there are a lot of pictures.
One of the links shows pictures of a drop-down tail-light housing with the gas filler behind it, which I think is probably the cleanest setup. The consensus seems to be that the license plate filler is too low to work well.
A couple of folks also put the filler in the top of the stake pocket on one side, which would work if you don't need to use the pocket for something else.