You can make one easily from popsicle sticks for near nothing! Or any two straight flat straightedges, metal, plastic or wood. jst match the angle and use a thumbsrew to hold them in plaace to keeping them from slipping, then just move them to where you need and presto! there you go!
So long as they do not slip or move the angle remains constant. You do not need any fancy graduated by degree tool though they are nice.

Look here, works great!

Robert