i actually use screens, i use metal ones do to their cheapness if lost and ease.

but if im out, or am too lazy to get one then i put a good sized little bit over the hole, then proceede to put crumbs, but not dust around that piece.

i would recomend screens, either glass or metal because it sucks to get a cherry in the back of your throat, and sometimes the hole is rather large and will allow the blocking nug to actually get sucked into and caught in the bowl without being smoked yet.