Syrup Proofing Feeders?

[LT]

I have some mating box in nuc feeders made of wood and masonite. Has anyone got any good ideas how to treat them to make them leakproof? I dip them in hot parifin but they only last part of one season. All ideas appreciated.

[Paraplegic Racehorse]

Silicon caulk at all joints. Three or more coats of polyeurethane on the inside.