You could add side shims, some have done that before, it's a case of splitting on the sides (if joined), expand to desired length, glue a piece of plastic behind, then glue a piece of plastic into the gap between the sides, you could then fill with ABS paste or automotive filler (bondo), the joins need to be filled as the sides don't have seams on screen seen armor.
ABS Paste
Shims (original trilogy with only one side needing to be filled, but will give you and idea)
Another here