I have a patio roof made of polycarbonate sheets. 1/4” from same tap plastic store in Berkeley. It bends when it is larger than 4 feet across. The sheet I got was a 4x8, so it bends and won’t stay up, so we had to put a wood bar in between and make them 4x4 squares, which are not bending. Why do you want poly for interior and not acrylic (which is more clearer, but is brittle)?