I was also disappointed with the flexibility - no fault of the instructions, but of the materials used. For the main light-tight material I used black curtain blockout from Spotlight. Excellent material for stopping light, as it has three spray coatings of polyurethane (I think). But sadly too thick. For the inner lining I used cotton lawn, also from Spotlight. The result was that the bellows will not stretch beyond 300mm, and I calculated on at least 360. And so stiff that I'd hate to use simple movements like moderate rise or shift with short-ish lenses. I'd say that for 8x10 or smaller cameras curtain blockout is too stiff, but it may be OK ULF cameras which need stiffer bellows.
Where to from here... I've thoroughly searched the offerings from Spotlight, and also online. I can't find anything online that isn't available at Spotlight. Trouble is that none of the other materials they carry are opaque enough, even when tested with a sample of lawn behind it. I'm happy with lawn as an inner lining, so have decided to next try an outer layer of ripstop nylon and spray the outside with flexible plastic paint applied after folding. Such a paint is available from auto spares stores and is specifically designed for plastic car bumpers to withstand bumps.
Why go to all this trouble? 1) I have a very tight budget for equipment and consumables; 2) I think it's good to keep alive these old crafts; and 3) I love making things, especially things that help me make photographs.
Cheers to all.