Have you looked into something that JB weld might offer.... sure one of their epoxy would fit your needs to seal the cup and not have the fuel effect it.@OTH Overland
I need to get a diesel tank cap with NO hole as I have been unsuccessful at keeping fuel from spilling out of the tiny vent hole even with electrical tape (the glue and tape seems to get dissolved by the fuel), and other plastic plugs.... gets loose with the terrain I have been on....
Does anyone know of a source to get the proper size cap? TIA