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 th