repairing blowout damage?

I have been up inside the under belly many times. Having to cut various access holes to get up inside. What I did was cut a 3 sided cut, about 15 inches per cut, I always cut so the uncut side is facing forward so no air can force the flap open. I would go buy 1x2 inch wood at Lowes and a small box of wood screws about 1 inch long. Once i cut the flap I would use the wood as a frame work to over lap the opening and screw the wood into the solid uncut belly material and then fold the flap up and screw the flap to the wood. using about a total of 8 screws per flap, sealing the cut up tight. If It was anywhere near where the cut flap would get water splashed up i also used gorilla tape to seal the cut area. Fast and easy. If your needing a bigger opening, no problem, just cut the wood to fit the opening.