Because each toggle frame will be filled with more metal pieces, making it quite thick, 22 gauge down to 26 gauge sheet backing can be used-do not use thinner sheet. Because the finished "O" ring will be bent in Step 43, the thinner gauge sheet may be desired. The size and shape needed is determined by the shape of the outer wall frame and the added the jump ring-see the photo in Step 15. Notice that for the ovals, extra length was added for the jump ring to be added. The corner of the square for the circle will be all that is needed for the jump ring. Check to make sure the size will fit both the frame and the jump ring before cutting the sheet. Use an ultra fine point retractable Sharpie to draw the size needed on the sheet metal. Draw a line on the sheet metal where the "O" ring opening will be cut.
Pro Tip: I have a tendency to make several versions of something I know I am going to like and might have small pieces or wire or sheet left after spacing things out to take the best advantage of the piece of sheet metal I am working with. In this case, there was a small rectangle of silver left at the end of one strip that was just wide enough for a small pine tree-so I cut two pieces and set them aside to later make a simple pair of earrings. The takeaway-think beyond what is being made as opportunities for new creations might present themselves.