I like using a Try Square. Sometimes I wish my small steel square could function as a try square, too. With the piece of wood shown in the photo, that is easily done.
Step 1: Begin With a Piece of Wood
Begin with a piece of wood 3/4 inch thick and nearly as long as the longest leg on your steel square. Make it a bit wider than the longest leg of the steel square is wide. Make certain the edges on the long sides of the wood are truly parallel to each other. One way to insure this would be to run the piece of wood between a fence on a router table and a straight bit in the router.
Step 2: Make a Kerf
Make a kerf about the thickness of the steel square. If the kerf is made to fit snugly, the piece of wood will stay on the square by itself. Make certain the bottom of the kerf is parallel to the edges of the piece of wood. This should be no problem if the kerf is cut on a table saw.
Step 3: Half-circle Opening
I used a hole saw to make a half-circle cutaway in the piece of wood. It should be located so you can apply pressure to the shorter leg of the steel square, in case you are using the longer leg to mark something.
For the setup I clamped another piece of wood next to the edge of the wood with the kerf. This supports the hole saw more completely while cutting.
Step 4: Kerf Across the End
For an extra touch I also made a kerf across one end of the piece wood so the square slides down into it.
Step 5: Use the Square
Here I have slipped the piece of wood over my steel square's shorter leg. My finger applies pressure against the shorter leg to keep it firmly pressed against the piece of wood I made and also against the edge of the piece I am marking. If the kerf is a little loose, a couple of small pieces of paper could be folded a few times and the folded paper could be slipped into the kerf with the leg of the square to function as a wedge that helps to hold the square in the wood.
The second photo shows using the square to mark with the shorter leg. Because the longer leg of the square is wider than the kerf in the wood is deep, my fingers can press the square against the wood at any point along the leg's length.