sub mounting question

in a sealed or ported box(made from 3/4 MDF frame for stability and covered in fiberglass). How are the rings where the subs go on kept without having the infamous fiberglass (curves) not allowing for that "perfect"seal to the sub ring? is there a special way of making the top of the enclosure? or is it just cut out what you don't need and sand around the edges? you reply would be greatly appreciated. It's better to ask a "STUPID" question...than to know a "STUPID" answer. :-)

hey matt thanks for the reply. i've been shopping for a new router and weighing out wether it's better to get a combo table or not. but thanks for the info. how much bigger should that ring be? another question...a local boatshop recomended the use of (sand-blasting glass-beads)...that material with the resin, and that will make a gelly-like substance that would be more beneficial for smooth sanding and finishing the visible surface of the enclosure. so before i go and waste the resin and have to scrounge up some sand-blasting beads...and then try to accomplish this "easy" task...is it a recommended approach? your help would be appreciated...and actually cost efficient. TIAIt's better to ask a "STUPID" question...than to know a "STUPID" answer. :-)