Used towels are just fine.
Layer and sew one towel on top of each of your prefolds. You can simply use a zig-zag stitch all around, trimming off any excess. Or, if you don't have a sewing machine, it is easy enough to hand stitch across the top and bottom of each diapers, using a running stitch, and don't bother trimming off the excess. Then place the coordinating dots along each side of your computer front, to match the ones on your piece of plastic. Then just line up and fasten one side, wrap around and fasten on the other side.