I removed and cleaned my tank last fall. There was already a clean-out port on top and all it is, is a round aluminum plate with many drilled and tapped holes. I went to NAPA & bought a sheet of fuel gasket material for $2 bucks and cut it to fit the plate. I would guess that the PO took a large hole saw and made the hole and then cut a plate to cover it. Seems that could be done for quite a bit less than $100 bucks. Will post a pic as soon as I find it.