Excess fleece

are the sides going to be shown? if they are there is a little more work involved. You have to staple the fleece down to the box on the sides as cloe to the edge as possible. The generously apply your resin. Then you have to go back and lay fiberglass matting along that same crease but on the inside of the box for added strength. Once everything is hardened and strong, go back with a vary course sanding disc or grinder and grind off the extra fleece with reisin in it to make that side of the box flat again.

If the side is not going to be shown, just staple the fleece down and resin it. dont worry abotu the way it looks.

There might be a better way of doing it but thats how I do it.

Cougar beat me to it!  Definately, rabbiting the edges is the fix before its an issue at all.  Also, as Nouseforaname said, bury the excess in the rabbited edge...so...

Add this to your mental data-base:  A box we built only required a small section of FG to curve deep into the box to feature a face of an amplifier and a 8 inch sub...together.  Kinda hard to explain the look.  Anyways, it was pointless none the less to fiberglass the whole side of the box just to make this indentation.  So, a rectangular hole was cut to allow fitment.  Then about 1 inch around each side of the rectangle hole I use a router to cut a "trench" as if it were a moat for the hole.  I used a

router bit like this oneto cut only 3/8 inch deep.  I stapled the cloth as close to the inside of this router cut "trench" as possible.  When I cut the excess polyester fleece cloth, I lay the excess that passes the staples down into the "trench".  I applied the resin to the cloth.  After the cloth is saturated, I pour the rest of the resin into the trench to fill it, along with the excess cloth.  When this is final it finishes off flush with the original surface, easier to smooth out overall which relieves you of the useless voids you have to sand, sand, sand....