I build a midhorn with 19mm MDF last monday, using 20º on the ends of the sides and 45º on the ends of the top and bottom.I had converted it all to metric and made a drawing in cad, that ended up making to mouth 63,5cm x 30,5cm, and the depth without mouting plate 26cm.
Now I have a pretty good idea how to make it the next time.
Which will not be in a while, I have to install floor heating and a new kitchen in the house, before I can do some more wood cutting for speakers.