[Aussie Mike]

Use some small fiber washers under the heads of the retaining screws. Cork gaskets can dry out as well. You could try making a neoprene rubber gasket. Neoprene is fuel proof, it's what a ot of the rubber parts in fuel systems are made from. You can buy sheet neoprene from Clark rubber in different thickneses and cut it out with scissors and a sharp knife. Use a wad punch for the bolt holes.

Cheers