Both of my hoods are plastic, with guide lines that CAN be used to make cut outs for your filter and heater. I found them to be too big for the equipment I had, so I just measured how big a cutout I needed, marked it on the hood, and sawed the hood down to the depth needed. Then using a straight edge (ruler) and a box cutter, I scored a straight line, multiple times, between the two cuts, and using pliers, snapped the piece off. No excess hole area to worry about. Everything fits snuggly, and it wasn't much work at all. Good Luck!
