^^^^^ now that could be done!!! Get a hood from the junk er, sorry, salvage yard and cut the under hood braces out and fit them to the area in question.
BUT!!!!! Since you already have that big sheet that covers the hole up, that piece is actually for safety, in the case of a hard rear end collision the fuel tank, that is the trunk floor, can/will rupture and become a fireball inside the car, if you 'dimple' that big sheet of metal in an X in the center that takes the flex out of it and it will not make that drum sound.
Another alternative, they make a piece that covers the top of the tank that isolates the tank from the inside of the trunk, I think it is/was called tank armour, you could possibly cut that piece you have down to cover the tank. Just put some stick on heat shield on the bottom and it should be good.
Just some ideas, either way I would isolate the cabin/trunk area from the fuel tank.