Recently I have been totally absorbed in transparent window stars AKA Waldorf Stars. On browsing one page, I noticed an origami folding mat which looked pretty handy for folding little pieces of paper. I looked around online and not finding anything else similar decided to make my own. I had to fuss with Excel a good bit to get it as square as possible, but I finally think I got it as close to square as possible. Print on cardstock for more stability.
(Click image to download .pdf file)