This is fantastic in the true sense of the word. If the disc is 4" in diameter, I'd buy a box of 4x5 sheet film and a circle cutter. The actual measurements of 4x5 are a little smaller, but it may work. Easier still, I'd construct the next version to take a full sheet of 4x5 film and rotate it around the center point. A faster film (TMAX400?) will let you stop down your lens.