Check these buggers out! No idea where to get them from but they exist. These waterproof nutserts would work great on the fuel tank, not sure if that seal will last but it doesn't necessarily have to with a thick cork gasket over the top.
Probably worth pointing out that the top face of the nutsert sits about a mm off the metal it's clamped through. But again, a thick cork gasket should do the trick.
If the area of the fuel tank that the nutsert is being inserted into is really thin. You could always find some really light sheet steel to double it up and just put black sealant in between the two layers. Nutserts do benefit from having at least 1.2mm to bite down on.
If you're worried about the nutsert spinning, you can always hit it with 240 grit paper to rough the contact area up, then coat with etch primer and put a dab of lock tight on to give it a bit more grab.