I found some pics I took during the construction of the bent wood horn for the imperial.

BTW:  I've tried to draw plans and 3D model it, and it's just too difficult.  Easy to actually build by comparison.



This is step one. Create a jig to define the inside flare. To create the actual flare, take a piece of 3/8 inch x 3/4 inch x at least 4 feet long. Hold each end of it and press it into an arc with your thumbs. Trace the arc onto the board. The two triangle shaped boards are going to become the sides of the horn flare. They have to get bent onto your jig.



Here is how you bend it. Once happy with the bend, tack it to the jig to hold it into shape.



Both sides tacked to jig.



The jig will be removed once the top and bottom are in place.  



To create the top and bottom, you will trace the horn onto a couple more panels. Cut the shape oversize by at least 1 inch. To get the top and bottom panels to arc up into the sides you will have to create saw kerfs (sp) like you did for the side panels. After the top an bottom panels are installed, you will trim them to fit the side panels.







The back of the horn (throat) needs to be square.  I set this one at 1 inch square.  In the past I have made these by drilling a hole to accommodate a screw-in compression driver.  It is important to create a smooth transition between the round hole you create (about 1 inch deep) and the square throat of the horn.  Lots of sanding.  In the pic above I found a plastic horn that I cut to fit on the back.  As I recall it did not work well.  So I just cut the threaded collar off the plastic horn and installed it on the back of the wood horn.



After everything is dry and the jig is removed, you then take Bondo and squeegee it into all the saw  kerfs to give you a composite, non-resonant 3/4 solid panel.





The final product is ready for a compression driver.  Hope this helps.  

What makes this horn flare unique is that the top and bottom panels are also arced.  This slightly compresses the air in the middle and improves dispersion.  An ideal horn for near field, ie. in your room say between 8 and 30 feet away.  PA horns are designed to project far distances in most cases and make lousy horns for the listening room.

-Steve

If the divider template were left in there, would the frequency balance change?
I have read that those old multi-cell ALTEC and Western Electric horns achieved two things with the cell structure. The dispersion was better because some of those dividers curved outward to direct the sound to the side, but also because each cell had a small mouth it carried or amplified the highs better than a single large mouth could do.  
    The two mouths formed by a single lateral divider are of course not so small as those of a 15 cell Mirrophonic or Iconic horn, but would they not emphasize frequencies in the middle of the treble band more than the single mouth?
    Whether this is good would I suppose depend on the usual variables of room, equipment, and listener preference.