I hope these pictures help. I'm not sure if it is the original way they were attached. There are aluminum pieces that serve as trim and the means for attaching them. You can see a small bracket at the top, in the picture showing the forward section. Mine attaches there, similarly there's a vertical piece in the back, and then there is an aluminum piece that runs the length of the panel across the bottom, which attaches the panel to the floor. Additionally, mine was fasten with bolts down the middle (about every 12" - 18". I can post more pictures if you need them.
I would mount the horn on the bow. If it's going to get in the way of canvas, I would go under the gunwale somewhere. It's a nice a horn and it's a shame to hide it, but the canvas has to work...