Well I just completed a full repair of my bellows on my Wista Field 45 I bought way back in 1977. I shaved it off from the front and back element as close as possible with a razor blade, lost one fold on both ends. Then stretched it out as far as possible so the folds were flat.
Then I took a large piece of thin black vinyl I bought from the fabric store. I sprayed both the felt side of the vinyl and the bellows with 3M 90 adhesive. Then laid the bellows so that one end of the vinyl was half one side, then worked the three sides so that the folds and vinyl joined as flat and even as possible.
When I got to the bottom part, I took a wide metal T square and placed it under the vinyl to act as a cutting surface so to not slice the bellows and folded over the top part, to not quite doulble cut the seam with a razor blade. I left maybe a quarter inch over lap, used another ruler for a straight edge.
Let the glue cure, cut away the excess and folded the bellows back into shape. The vinyl acts like a veneer over the old one and sealed it light tight.
It folds as good as new.