stepside fuel filler/cap options

[19GMC63]

hey guys,
I'm about to install the poly blazer tank kit in my 63 gmc, I know fuel filler ideas have been covered on here many times before, but my idea is a little bit different than the ones i've seen, and I need some help, and thought i'd try you 67-72 guys.

Basically, what I want to do is put a flush mount cap on top of the stepside fender, or possibly on the back. I want to keep the side of the fender clean, and I don't want to put it in the bed. Too bad I don't have sidemarkers.

So my questions are, has anyone done this, and/or does anyone know how I can go about doing this? Pics and links would be great.

Thanks for reading.