I would make a form from thin plywood and furring strips. Make a frame from the furring strips and then fold the plywood over it. The trick to folding plywood is hot water. When you wet the plywood with hot water it will go limp. Then place it over the frame and attach it to the furring strip frame with wood screws working it from the centre out to the edges. When it's dry use contact cement to attach felt to the plywood where the Lexan sheet will lay on it as you heat it. That will prevent scratching the Lexan. You'll be pleasantly surprised how easy it is to do. It will be easier to work with than the hot, wet, plywood was when attaching it to the furring strip frame.