I have used (used) dryer sheets and phone book paper. When I need the center strip to be exact I will use paper (any kind but more recently found that the paper that comes from annual stock reports is bigger and works well). When I made my boxes of strings. I only sewed on the paper 2 seams, the first seam on either side of the triangle piece, Then I folded back the paper out of the way for the rest and then I only had to pull off the paper from two seams. Easy peasy and I was able to use small strings and bias, what ever worked for the color I needed. By having the paper cut to the size of the block I was making I could check that I had a big enough piece to cover the paper and therefore big enough for the block. I hope that it makes sense. Now I do all my string piecing this way, only sew the first 2 seams in the center. Funny though, I still save the dryer sheets!