Cutouts on Fiberglass

I am going lay fiberglass soon to act as a flat surface over my sub box in my car. Supposing I wanted a cutout shape in the fiberglass surface, what would be the best way to go about this? I would think that attempting to cut through the dry fiberglass would be a bad idea, especially if I wanted my shape to be precise. Should I try cutting out the shape in the fiberglass matte before I try to lay it and add resin? It may work if I do the same shape in each layer, then match it up perfectly. If I got it as close as I could that way, could I then use a wood replica of the shape I wanted, (covered in duct tape or somthing so that nothing will stick) to KEEP the shape I wanted, while I filled in any bad fitting around the cut shape with body filler? After the body filler dryed, I'd be able to pull the wood shape out, and the surface down nicely. I know my explanation is horrible and doesnt really make sense, so I will explain my theory further if necessary. But otherwise, would this work at all, or does anyone have any ideas? Thanks

wood idea and try some wax paper i dont think the fiberglass would stick
but i would leave the wood to keep it strong
what are you trying to make

try to post it here
http://www.automotiveforums.com/vbulletin/forumdisplay.php?f=1266

cut it after its hard. fiberglass is just like plastic once its hardened, u can just use a jig saw or similar to cut out whatever u need. Word of warning, wear long sleeves when cutting fiberglass, the shavings are VERY itchy.

i think i have it figured out, i will post when i am done with it. it will be a while though, i dont have much time to work on it.