At the prodding of another forum member I began looking into the Smith Selsted tweeters from this post:
http://audioheritage.org/vbulletin/s...ead.php?t=9871
I took the small sketch in the article, saw that the diaphragm is 1 in and scaled up to see what the size would be. It turns out the height of the horn scales to about 1/2", the same size as the opening on the 2404 ring radiator. So I cleaned up the sketch, made a pattern and duplicated the horn in walnut. Here is the result.
Initial listening tests are very positive. My RTA unit is pretty worthless above 10khz, so I didn't even pull it out.