What can I waterproof a car remote central locking fob circuit board with?
Usual way we make sure designs are sealed even if they have gaskets is to run a bead of normal gutter silicone along the seam on the edge, however in your case you would also want to cover the led light duct and the rim of the buttons, 

This way its not too difficult to reopen, but if you clean it back to be smooth to the surface will stay put, 

Have mainly only done this on outdoor monitoring systems, so not too sure how much other things in your pocket might hurt this,