They are called a "bevel" and used in various trades. Boat building for one. Anywhere it is important to transfer an angle and not necessary to measure it.
Here in thailand most welding shops will have one made from scrap steel.
Forgive the ASCII drawing but they look like this
Welded Bolt with wing nut | | V V
--------------------------------| | -------------------------------------------
--------------------------------|
Three strips of, say 1 inch X 1/8 inch and a 3/8" bolt.