This is the one that I use. It's customizable, so you can input the exact size of your washers and letter stamps and get one that fits very well. The first one I tried, the hole for the stamp was too tight, and using it was a pain.

I also modified it a little from the original (added more letter slots for longer words) so I can use it to make ID tags for the trees in my orchard. Smaller trees, you can hang the washer on a limb with wire, and when they're big enough, attach it to the tree with a stainless steel screw.