Can't think of anything that wouldn't also ruin your cutter. I got a 6' ruler at Lowes. When I want to square up a quilt, I lay it on a table, use a large square ruler to mark the corners square, then use the 6' ruler to join the markings between the corners, along the edges. With the quilt marked where you want to cut it, you can slide it along your mat, cut that portion, then slide it down more. You don't have to worry too much about marking the quilt because the edge is going to be covered with the binding.