I’ve replaced the stock Mustang fuel tank with a 16 gallon Jaz fuel cell (#271-016-01) - it has a polyethylene inner shell encased within a steel container. I ordered it with optional safety foam and a roll-over valve flapper under the cap. I made a basic frame out of 1" square steel tubing which bolts directly to the cell and is then bolted to the trunk floor by the frame rails. The fuel cell is offset 3" from the stock fuel tank location so that it is centered (for some reason Mustang tanks are not in the center of the trunk) - it tucks in nicely without having to cut into the floor and sits roughly 2" beneath the rear valence which makes it just visible from the rear. The space around the fuel cell will be enclosed with aluminium panels.