The KX'S come with a foam gasket,they call it a duct dampner between the airboot and metal rim that your filter mounts to. The kx's have a good design, when you hear to seal up the airbox it really isn't the airbox you are sealing up.
If you look at the outside of the airbox, right where the airboot mounts to it you will see a metal ring mounted with nuts.Take the nuts off and pull the inside metal rim off. You want to seal the outer edge of the inside metal rim with silicone and bolt it back together,but like I mentioned kx's have this covered. If you still want to silicone it, go for it .
No need for the pro seal just take the white plastic filter holder without the filter and mount it on the metal rim. You should have a nice snug fit,now imagine the airfilter between that.