Ben, you don't need to form the piece for the back window/engine cover. It can be trimmed from sheet and takes the shape of the car when screwed down. I don't use lexan as it scratches even when wiped with the softest cloth. I use 100% acrylic for the headlight covers I make since it can be wiped with anything (except gravel) and it doesn't scratch. They do make hardcoated lexan called MR10 (or something like that) that is more scratch resistant but it is 4 times the price of acrylic. In my experience, the heat of the engine doesn't affect the plexi and cause it to sag. Due to the memory of the material, cool areas will always pull heated areas back into shape. That's why you couldn't reshape your old piece. It must be "submerged" in heat, such as in an oven, to fully reshape. This has been my experience anyway.