The first few horns I tried didn't turn out very well. Aside from the fact that 3/4" plywood isn't the easiest thing to curve into the walls of a horn, they didn't sound right. Later I found a free horn simulation program called hornresp. When I plugged in the speaker parameters and the horn shape I had used, it predicted a broad peak in the frequency response around 300Hz. I have heard the ears are especially sensative to peaks in this range.
With this new tool I tried again. I found that tractrix horns seem to give the flattest frequency response if they are calculated for the speaker's resonant frequency. I had a pair of full-range drivers that looked quite good with a 140Hz flare.
I saw a site on the Single Driver Webpage that showed how to make polygonal horns out of masonite. This material has a few advantages over plywood. It curves rather easily, it doesn't resonate too badly, and it is cheap.
So, I calculated the curve for a tractrix horn with a cutoff frequency of 140Hz, truncated to 1/2 full mouth area. Then I converted from the radius of a round horn, to one edge of an octagon with the same area at several points. Finally, using a bit of geometry, I converted the horn length to the length of an edge. I plotted these points onto some sheets of graph paper taped together to make a pattern for each side of the horn. The pattern looked like a long skinny trapezoid, but with concave long edges.
Using my pattern I cut out eight pieces of masonite for each horn. I taped the eight pieces together at each corner. This produced an eight sided object that was roughly cone shaped with curved gaps at each edge. Working down the horn, I forced the gaps closed and taped the sides together with a combination of duct tape and glass straping tape. Once the gaps were fairly well closed, I ran a bead of polyurethene glue down each edge. This foams as it dries, sealing any remaining gaps in the edges. In this application, I don't think it could bond very well with the masnoite, so I left the tape in place.
I made a flange for each horn out of a small piece of plywood. I used a scroll saw to cut an angled octagonal hole in the plywood to fit around the throat of the masonite horn. The driver is mounted to this flange with screws. The back of the driver is open. I currently keep a blanket draped over the back of each driver to absorb some of the sound from the back of the cone.