You want to use dimensionally stable polyester-based sheet film for the strip as well as for the masking film itself. These are carefully taped together on the long side using an appropriate mylar or polyester tape, which must likewise be immune to humidity and temperature dimensional changes. There is a distinct technique to re-register things accurately, involving not only a register frame matched to the specific punch involved, but something flat and smooth to press down on the sandwich when taping it over a lightbox. I use a smooth polyethylene Bondo applicator. Narrow thin mylar graphics line tape works well for the tape itself, but it's getting harder to find in art stores, so web ordering seem likely. The main problem with 6x7 film, and all roll films, is that they are thin and squirrelly. You want to have your chosen frames removed from the roll and pressed for awhile for optimum flatness. And stable reasonably-dry conditions are favorable. Acetate films change dimensionally with humidity changes, faster than you might think. But the net dimensional change in most roll film frames is minor because the film is so small itself overall. Yet I must confess that roll film is a misery to keep in register over the long haul compared to Estar sheet films, but I frequently do it and am now perfectly comfortable doing so based on sheer experience.
