Wall Thickness Caliper for Carpenter Square

I have an old house and am creating some CAD drawings of it to help plan a couple DIY projects. I needed a simple way to accurately measure the thickness of my walls around the door jambs. I had a carpenter's square handy and made some jaws to turn it into a wall caliper! It turned out to be pretty effective; I can just subtract the distance of the closed jaws from the reading I get when measuring.

The dimensions of the slots are tuned to my square, and your mileage may vary with regard to the fixed jaw staying in place and the sliding jaw moving freely.