If you are "handy" with mechanical stuff, you can find boat fuel tanks that are flat. I have been toying with a very similar idea using a stainless steel boat tank.
2004.5 GMC SLT CCLB, Ride Rites, Turbo brake, EFI Live
2012 Arctic Fox 29 5T 5th Airborne, B&W, Prodigy
Papillon navigators Lily, Buddy and Willow