I use thin muslin for my backing, cutting the squares 7 1/2 inches and cut them into 7 inch squares when done. I went to the auto and window glass near my home and had them cut them out of plexiglas--very reasonable. I'm not sure what width your strips are but I'm estimating seven strips will fill the block. I usually cut 3 or 4 inch squares, cutting them into triangles for the end pieces. When setting the blocks together, I use a 1/2 inch seam because I like to press the seams open. For me, making string blocks is addictive.