I get what you are trying to convey I think Pawned.
However, that wouldn't work so well as that is the point of the adapter. *To space it off the quarter panel ever so little so the cap doesn't touch the sheet metal.
If you look at the cap base closely, it has a bit of curve to it, that will not sit well on our flat quarter panels.
Also, if mounted between the quarter panel and the filler tube (inside the trunk), you would need to shove the filler tube farther into the tank and most may not like to mess with the filler neck and disturb the seal at all.
I appreciate the thought and suggestion though! Make sure you post pictures of it installed if you dont mind! :)