Just brainstorming here but have you thought about using 120 film taped into a 4x5 holder? You could tape a strip of 120 in the center of a film holder and then have an "dark" darkslide with no cuts and a "active" darkslide that is cut in the center to the dimension of the 120 film. You could get just shy of a 6x12 negative that way without the cost and compatability issues (your camera has to have a Graflok etc..) of a 6x12 back.
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