I am installing a new three piece shower in my house. I left the shower stored in the box for a couple of years before I got to putting it in. The two upper pieces have distorted in storage so they don't fit the 90 degree corners anymore. When I set them up the middle of the back wall where they slide togeather is sprung out a couple of inches from the wall and there is no way to screw it down and keep the screws hidden. I have had it in place for a while, screwed on the edges and a brace pushing the the center of the rear wall back, hopping this would correct the problem. It has helped but not enough for the correct fit. Would I dare use a heat gun to warm the fiberglass? Any other ideas on fitting?