I have also made a sound post gauge from coat hangers and it worked just fine. The thing that people get crazy with is the angle on the ends of the sound post. The angle on the outside of the instrument is the same as the angle on the inside. Just set a piece of post on top where you want the post to be and sand down the angle on the post.  Take your measurement and cut the post then set the angle on the back of the violin where the post should be.  I have a violin without a soundpost so I'll make up some pictures and dirrections.