Do you have a vented or non vented cap? There are two types of vented caps:

1) Through a hole in the top
2) Through the threads

Either could be tough to seal. There is also a non-vented cap. My suggestion for sealing the non-vented would be to remove the rubber gasket the cap seals to the top of the tank with, and coat the entire underside of the cap and the outer threads of the tank with epoxy and screw together tight.

The other option is to tank the tank to a plastics place and have them weld a piece of plastic over the hole where the cap screws on. They can do a spark test to make sure it's air and liquid tight. We have a local place(40 miles away, considered local in CA lol) here that can do it. Should only cost $10-$15 or so to do.