I just completed a DIY bag bellows and tried it for the first time this morning. I used a set of plans for the bag that I found on the web (
http://www.foto-net.de/net/dyo/gross_ww.html), while the frame was duplicated from the frame of the standard bellows on my Zone VI.
I used a vinyl fake leather outer layer and a felt inner layer (per the web plans) - conceptually, that's OK and probably would be fine with a monorail camera. But its pretty stiff and makes it difficult to install the bellows on a field camera (where the bellows is installed from the rear and must fit through the rear standard). If I had it to do over, I would use something more flexible - perhaps a couple of layers of black velvet.
The bag was stitched by hand using needle and polyester thread.
The frame was made by glueing up strips of maple to the right dimensions. A key lesson there is "measure twice, cut once". (Don't ask!) I used polyethelene glue - it worked very well.
I attached the bag to the frame using two-component epoxy. I used some scraps of the maple (some of the material that was only measured once) as a caul - wrapped it in polyethelene so that the glue wouldn't stick, and then clamped it with spring clamps while the glue dried.