If tank top is wood with glass skin, the top surface is probably not smooth enough for a gasket and al plate. In this case, grind down surface and mix an epoxy putty and squash down with a flat plate and plastic sheet as a release. Once it kicks, lift the plate and you now have a flat surface for gasket to seal. Cut hole and seal raw edges of wood with epoxy. Depending on how thick tank top is, you can use lag screws or wood screws that do not penetrate into interior. If thin, you can use a variety of fasteners but they will need to be sealed. Any fab or machine shop can make you a cover plate out of al scrap like 6mm for low money.
Put a center tap hole in plate so you can dipstick tank level, or dip a cleanout lance without removing plate.