The outer material is a rubberized nylon fabric which Porter's Camera Store sells as "darkroom cloth". The inner liner material was 150 denier polyester matte black "Supersuede" from Outdoor Wilderness Fabrics (owfinc.com). Stiffeners were cut from black posterboard and I used black foamcore for the form. I used automotive headliner spray cement to attach the stiffeners to the outer layer and 3M automotive trim adhesive spray to attach the inner liner to the stiffener/outer layer. DAP contact cement was used to cement the diagonal seam on the underside of the bellows. (Buy plenty of spray adhesive if you plan to try this. I used 2 full cans on this bellows.)
It actually went together fairly easily once I figured out how I was going to glue it up. You need plenty of horizontal space for layout and construction, but otherwise, it was fairly easy and certainly worth the investment. I probably have about $60 into it of which $30 was the cost of the adhesives. I also used a fabric pencil, some masking tape, and a lot of paper towels to cover the surfaces when applying the adhesives. I figure a custom bellows this size would probably cost triple what I have into it. In addition to monetary savings, there is also the satisfacton of actually making it by hand.
It took several hours to plan the bellows and draw the layout on the materials. Cutting the stiffeners was maybe a half hour and gluing them in place probably took an hour. At that point I let it sit overnight to set the adhesive. Applying the inner liner and then folding it took a matter of minutes to complete although I did let the attached liner sit under weight for about an hour before attempting to fold it.
Now that I've done it and know what to do, I could probably make another bellows this large in a couple hours. Next one will be for a 5x7 Seneca Black Beauty and then perhaps the 8x20 Korona...