I teach a class on hand cut dovetails and in that class, I have my students use a small square to mark the tails. I like double squares and needed a couple more for the class so I ordered two of this one from Amazon. While it's inexpensive, I found it to be a good product. Also, it was free shipping. If you're looking for a small (4") double square at a reasonable price, I can recommend this one.
I am in no way affiliated with the company who makes or sells them. Just wanted to pass this along.
Mike