The film holder/film gate shown here: Attachment 57459 is the second (or possibly third) version I made. I still have the first version which uses some parts from an old folding camera to hold the film, rather than the FR4 springs I subsequently used.
You are welcome to that if it is of any use to you. I can't post a picture of the actual item until tomorrow as it's at work - and I'm not!
It's also worth looking at the Holga 6x12 pinhole camera as a cheap donor for a 6x12 back.