Simple solution
I assume that your access lid will overlap the size of the hole. If so, you must screw the lid down around the outer perimeter (where the two surfaces meet). Drill a hole in the lid for each screw to pass through and tap the tank with the proper thread to secure the screw into. Then you can go to an automotive (or good marine) store and pick up some cardboard-like gasket material for gas or diesel. Cut it to shape, put holes in it to match your screw holes and install it. If you want more, you can go to the auto store and get some caulk for fuel purposes as well.
