In my 8x10 kit I carry a 4x5 reducing back and wooden splitters that fit inside the 8x10 spring backs on my Kodak 2D and Century Universal that permit 4x10 and 5x8 formats. I love the versatility this gives me--four formats with one camera. Recently, I decided to try to add a fifth format--7x11. I purchased a vintage 7x11 back from Jim Galli that I thought would be a good, vintage match for my older cameras. I knew the 7x11 back would need to be modified. I wasn't sure how much of an extension I would need to avoid vignetting, but I thought it was doable. Thinking that somebody else might be interested in trying this, I am posting to share my experiences in adding a 7x11 back.
When the back arrived from Jim, I was delighted to measure the depth and discover it was over an inch thick. It was also 11 and 7/8 inches square (o/s), and my camera backs are 11 and 1/2. I took the back to a woodworker who has helped me with vintage camera mods in the past and had him trim the top and bottom to 11 and 1/2 (I decided to forgo portrait mode for the time being). I went back a second time and had him trim out the inside to a true 7" x 11" since the original opening was quite a bit smaller (about 1/4" each way).
To make the back usable without adding a cumbersome 2-3" thick spacer, I filled in the rabbet and added a new light trap myself. Pictures below.
W/o any additional spacer, the back is usable with 2-3 lenses: a 159 ExWA Wolly, a 210mm Fuji (these two do not vignette even when wide open), and I think I can use it with a 300mm when stopped down (need to verify this last).
First pic below shows the back w/o gg after trimming. Second shows detail of the top/bottom cut. Third shows detail of the inside opening.