Carl Goldberg used to include one with their Ultimate biplane. I think I still have one I see if I can find and post a picture. It was just two pieces of flat ply and a U shaped bracket for each end you glued the flat pieces to. Then stuck the paper on. They work pretty good. Overall it ends up about 4" long and 1-1/4" square. Every kit had one, maybe someone close to you has one.

David