I've been looking for the metal ruler too. Walmart did have one but it was metal with paint on it so I passed on it. They do make metal rulers called "squares" that I'm sure would work fine. They are more of a triangle but have straight edges so it should work. Right now I don't use anything with my wood burner and I'm doing alright.
__________________ Amanda~ Mama to 2 Free Spirits, 2 fur babies, and 29 fluff balls.