I've made a few bag bellows...coincidentally for 5X7 cameras! I use a four panel design that seems to work well. Basicly you need to figure out how much max draw you need...that plus 25% will be the length of the panels. The front standard height and width plus 1/4" on each side will be the front height and width that you'll cut. You need the 1/2" on each dimension to glue and stitch the 4 panels together. The back follows the same plan. The maximum width of the panels I place about 1/3 to 1/2 of the way from the back, depending on the type of camera. The width can be enough to fit inside the rear standard of a field camera, or much larger. The wider the widest point, the more possible movement, but lessens your max draw. Each panel should be an elongated hexagon when cut. I use two layers of material on each panel...the lamination can be done either before or after you cut the panels. Then just glue and stitch the edges together while inside-out. You can make up boards to fit the front and rear standards that make interchangeability easy. Paint 'em flat black & attach each end of your new bellows with glue and a strip of wood beveled at a 45 on each end. Voila! They usually take me a couple days of casual work to complete. That was an explanation about as clear as mud, I bet!