FINISHED!
Here it is with the 90mm lens attached (though it really is mainly for pinhole use, it will use lenses when I want it to).
I think the weak point is the bellows which is pretty exposed most of the time and is not attached very securely.
It was an absolute *BITCH* to try to attach the bellows - totally horrid experience. I ended up attaching it with spray adhesive at the front, which turned into a messy nightmare (even though I'd masked the front's non-attachment points with masking tape), so I went with wood screws (and the pressure from an internal frame) for the back. This attachment isn't very secure, unfortunately, so I might unscrew it and add some glue. As it is there's the risk that corners of the bellows will pull out and then it won't be light tight anymore.
The other problem is that the bellows pulls inwards when I go to full extension, possibly blocking the light path inside the camera. This is because I glued the bellows together around the edges then turned it inside out, so it has a natural tendency to fold inward. Oops.