I need to replace an existing pivot shower door and frame. The aperture is 780 mm but i can only find on sale 760 mm and 800 mm wide door/frames. Do I buy a 760 mm and pack out one side with a batten OR buy an 800 mm and try and cut down the frame...? OR will the 20 mm adjustment for out of true walls stated against the 760 mm frames give me the 20 mm I need..... The existing is 20 + years old and may well have been bespoke back in the day. Any help much appreciated please?