Why not build a pair of them? The main structural support for the cab is in sides, the back, and the speaker baffle. The port is made by a piece of wood inserted in a dado groove. The curved horn structure can be made of a piece of thinner, more pliable wood ( 1/8") - in fact, I don't think that the horn sides even need to be wood. The material that forms the horn doesn't need to provide any structural support...it just directs the sound. If you look at the pic you can see the screws that hold the horn in place. The mounting rail behind the horn sides into which the screws go is curved. That should be easy to do on a band saw, or with a jig saw and a radius bar.
Sunn also made a similar cab where the sides of the horn were straight...not curved like the cabs you show in the picture. They might be a little easier to build and smaller too.