a local hardware store or home depot store should be able to supply you with plexiglas or other clear plastic. this can be cut into squares of different inch increments. with a fine-line permanent marker, draw diagonal lines and cross lines at the center of each side. that will give you the center of each block. mark each one permanently with the size unfinished: 5-1/2. 8-1/2, etc. that will provide the seam allowance. you can square up any size block very frugally.