Here's a link to the best discussion I could find. I plugged the formula at the end into an Excel worksheet and came up with about 100 degrees for the angle where the sides meet. That's just what I came up with when I custom-fit the sides into the top and bottom. I imagine if I could end up with perfectly mitered edges and beveled front edges, the horn could be assembled by placing all the pieces in a pyramid and a little piece of tape on each edge would hold it perfectly until the glue dried. Right. My reality-based approach involves deck screws and wood filler.