The last two lap quilts I made had "crazy quilt" backs made from the scraps from the front. I simply continued to add pieces, squaring occasionally until I reached 12" squares then made rows of the 12" squares and adding whatever I needed to the end of each row (off setting each row with the odd measurement). One row would be four 12" blocks with a 3"x12" block at the end and other I'd reverse the next row to start with the 3"x12" block. That way there is a sense of order but it finishes like a crazy quilt look and no backing material was needed.