That's probably what I will do. I do have some lightweight black opaque construction paper that I think could also work as it seems about the same weight as the real backing paper but I'd have to find it in roll format.

As far as a pattern, I did just win an *Bay auction for an Instamatic 104 with 2 126 cartridges (Kodakcolor II) and paid less than 5 dollars. The film itself is likely worthless but the carts and backing paper will be useful. There are also lots of 110 cameras out there with old film included, too. Many of these camera outfits appear to be unused, NIB.