exactly. Pick up some galvanized or ss sheet from the local hardware store. Next time you drop your tank, eyeball where to cut, with the tank dropped drill a large hole and raise the tank up and check position. Once you have that defined, use a cut off wheel or big tin snip to open up a whole big enough to access the fuel tank hat.
File off the sharp edges and then shape your sheet steel for the opening, leave enough material around the hole to seal. using a ball peen hammer to contour the sheet to the space. Due to the clearance there you need to be cautious with fasteners. I'd be shopping the hardware store and probably use some sort of hinge setup for the new door. Then seal it with fuel resistant RTV.
I wouldn't try to do this with the tank in the chassis, since there really is near zero clearance between the sheet metal and the top of the tank.