hi dan

you can probably get a 6x9 roll film holder ( for a lf camera if you have one )
and dremmel it out to fit your desired aspect ratio the problem will be figuring out how to wind the film so you get what you want ...
seems like a lot of trial + error . you could also get the same roll film holder and modify a darkslide to cut the film in half
and flip it to get 2 2.75 x 9 views on each 6x9 exposure ... ( less trial + error )

jeff's suggestion of daYi back sounds the best ...
i wish i had one !