Slightly smaller film width will most likely work in most cameras. However, the Exeter paper I use for backing needs all the width available to ride properly on the curved back of the #4 Panoram (size 103 spool). This paper does not have the exact characteristics of regular backing paper. If I understand you correctly, the modular system could work for a number of film sizes, but the ends of some obsolete spools have different diameters, such as fit the Graflex roll film backs in the #'s 53 and 51, IIRC. There would have to be accommodation for the tops (backing paper leaders) to be inserted in a slot in the middle of the spool, but I am sure that could be sorted out.