Back the gaskets up with dum-dum. People who don't know correct automotive vernacular may call it strip caulk.
Something that is almost as good as dum-dum, and will work fine in this application, is stuff called duct compound/sealer/pukey whatever. It comes in a small brick that costs a couple of bucks. Cost wise, the same amount of dum-dum costs about $20.
How much and where you place it requires common sense, something I cannot teach.
