37-degree included angle, 18" axial length, and a 1.375 throat to match the driver.

I am planning on 1/2" birch ply-wood for the construction. 8 petals, with a 22.5-degree chamfer( 'cause that is what is available from the router bit salesmen ).

Considering how well the experimental version sounds, these ought to be Sa-Mokin'... The experimentals were Fostex FE107's in 30-degree 6-petal 17" horns. They took a single afternoon to create with help from Home-Depot's saw man, and 5-minute Epoxy. 2" throat on those.
cheers,
Douglas