Oh, I thought that this was a permanent mod.
fibreglass is a very forgiving material. Try not to go through the shell, but repairing fibreglass amounts to shaving each side of what you want to connect to a point that starts about 2" from the edge. If you can get to both sides, shave both sides. Clamp it in place and add a 4" wide strip of resin soaked fibreglass mat to both sides of the seam. Sand smooth. Add more fibreglass if necessary. Sand again. Then paint
There are threads on this site where people have repaired crushed trailers.