Here are my current bellows panels:
This was a little difficult to work with, because the fabric doesn’t hit it at a right angle. it’s much wider at the bottom than the top. You can see in this photo how the top of the bellows is really more like a hinge, and the bottom is where all the expansion happens. You pretty much place the panels onto the fabric at this angle, and then glue them on. That means at the bottom in particular, there’s a very small contact patch with the panel, unless you really work the material to stretch it around. The top has a similar issue.
It seems obvious in hindsight, but the solution is just to skew the panels.
I checked how David had cut the panels on the bellows I bought from him, and of course there was the same skew I’d just realized I needed.