The best sealant I have ever used on cases is silicone.... the stuff people love to hate. Silicone, like anything else, has to be used correctly for the application and this is the reason folks bad-mouthed it because they didn't use it right!
Naturally, have very clean mating surfaces when you start.... a good wiping down with lacquer thinner or such. Spread a THIN even amount on the joint.... oh credit card thick. Assemble case but only tighten the screws gently until you
see the slightest amount of silicone bulge out evenly around the joint. Tighten no further! Now go away and let the silicone fully cure.... at least 24 hours. Now you have a really good strong rubber gasket there instead of a squooshed out mess!
It is now time to fully tighten the cover.
Naturally, have very clean mating surfaces when you start.... a good wiping down with lacquer thinner or such. Spread a THIN even amount on the joint.... oh credit card thick. Assemble case but only tighten the screws gently until you
see the slightest amount of silicone bulge out evenly around the joint. Tighten no further! Now go away and let the silicone fully cure.... at least 24 hours. Now you have a really good strong rubber gasket there instead of a squooshed out mess!
It is now time to fully tighten the cover.