Well, that didn't work - the wood cracked again so I tried with another piece of mahogany - this time with nice straight grain (I figured that being on the inside of the horn no one would notice, besides that's the only choice I have). This time I used my heat blanket instead of the hot pipe
The piece came out good and I fit it into the rim. There is a lot going on here and unfortunately I didn't take pictures of all of the steps
There is a wedge shape block that forms the point of the horn and the part that fits against the neck block has to be exactly the correct width - that will flow into the neck heel and needs to be exactly one half of the width of the neck at the 14th fret. I fiddled with this a lot before gluing up - but forgot the camera laying on the work bench.
Here is the rim with the clamps off laying on the original ES-175 plans showing the change in shape of the cutaway.
In a way I like it more than the really sharp pointed horn (my wife said its "not as horny". I e-mailed that picture to Jesse with a brief explanation for his approval and he said it was OK.