This is a great use for old bath towels and the only cost is in the purchase of gridded matting or blank latch rug canvas with grid.
All you want to do is cut the towels into strips of approx. 3/4” thick, and then cut the strips into 5-6” long pieces. If you do this, you will need 2-3 old bath towels for a rug sized 18 inches by 2 feet. Then knot each strip along the grid, ensuring they all face the same way.
For a thick, full mat, knot along every line. For a thinner mat, knot along every other line.