I made a set bag bellows yesterday and it actually turned out pretty well. WA bellows can be difficult to find, especially for custom projects, so the here's the link to the blog post if anyone is interested in building some. Only took a couple hours.
Downside is the BK-5 cloth material is pretty expensive. It also needs to be doubled up to be completely opaque, but that actually helps give the bag some rigidity so it doesn't settle into the frame mid-exposure. It's still plenty flexible for movements and will fold down flat for storage. (Doubled up the material is probably a little too thick for pleated bellows.) The minimum order from ThorLabs will make quite a few smaller-format bag bellows. The fabric is surprisingly durable- I made a similar set 4 years ago for a different camera, and it's still light tight after some pretty rough usage and storage in my pack.
The other materials are pretty basic- some rip cloth between the BK-5 really helps with the adhesion, some spray adhesive to laminate the materials, and some double-stick tape for the seams and attaching to the frames. McMaster-Carr sells a good quality tape. They call it light duty but I think that's relative to the structural tapes they sell, it's very high tack.
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