I own and regularly enjoy a pair of Yuichi 290 horns driven by 2445 drivers with the Brush Wellman Be diaphragms.
I bought the Horns from a regular contributor here. He did a great job on the build. The transition piece he made is well done but not exactly as smooth as I would like as it transitions from circle to rectangle. I recently ran some 3/4" Baltic birch plywood to come up with a smoother transition.
The Yuichi has a rectangular throat that is 2.25 x 2. I simply lofted between a 1.95" circle up to those dimensions vertically 3/4". I'm very happy with the results from a physical standpoint as both sides mate with their respective components with virtually no play.
For some reason, I was convinced Yuichi had used a 3/4" transition piece. But no, my memory fails again.
Yuichi used a 1" piece on his design. If someone might chime in and tell me if that 1/4" will be signifigant, I'd appreciate it. It would be some work to make it thicker but not radically difficult.
Any other comments/suggestions on how to improve upon the design are very welcome.