Screwed in types and have a thin light seal material under all the edges and are the best of all systems.
MOST n best glue.... Non-screw in types use contact cement, spread it on both surfaces n allow to dry before sticking em together.
Some non-screw in types use double sided tape but I don't care too much for that, not a secure feeling about it.
Some DIYers use silicon but I don't like that as it tends to come lose as it ages n is messy if you use too much.