If you stretch strings from corner to corner on a square or rectangular frame they will cross at the center. Before securing the top and bottom on the torsion box you need to make sure the frame has no twist. Secure one string tightly in place at both ends making sure it contacts the frame at each end. The easy way is to have a screw just down from the edge for this. Now secure the second string the same way at one end and pull it tight across going over the other string. The second string will either touch the first string or will be above it. If it is above that means the corners it is tied between are above the other corners, i.e. the frame is twisted. If it touches loosen it and pass it under the first string and pull it tight. Again it will touch or be below the first string. If it has touched both above and below your frame is not twisted; if it is now below and not touching the corners it is stretched between are below the other corners.