John,
(there was a url link here which no longer exists)
and
http://www.galerie-photo.com/porte-film-jobo.html
The first is a link to a thread where I described my experience with a DIY sheet film insert, although a bit smaller (for 5x7 negs) - but the principle is the same.
The second link is the original article (in French, but with lots of photos, so it' easy to follow) which gave me the idea.
As for the materials, the holder is made of construction-grade translucent plastic, found at the local equivalent of Home Depot - must be some window screening stuff or something, it's designated as "UV-protection" or something. But, it's not a mesh, it's solid sheet of PVC. I used hot air gun to make "dimples" to prevent the negatives sticking to the holder.
Pins I make from whatever is at hand - mostly plastic plugs, etc. Keep in mind the material has to be water-resistant - you don't want rust in there...
BTW; I also have the original JOBO sheet film holder - but it's actually not a sheet film holder - they call it "JOBO Formathalter/Format holder" and I think it was made for developing test strips. JOBO code #4206, and it's small - you could probably fit just one 5x7 neg onto it.
HTH,
Denis