I use 1 1/2" screws, but Ive got a 3/4" ring of mdf on my box so I've got an 1 1/2" of mdf to screw into you aren't going to want anything longer than the thickness of your material.
the cap stays on because it job is store and release power quickly to you amp, for cleaner faster response. leave it in, it stays on because the current has nowhere to go.