I suspect there are a lot of things in various stores that would make good phase plugs. Model rocket nose cones come to mind. Right now, I'm looking for things that will work for a 2 inch voice coil. Sometime in the future I'm going shopping for a tweeter that will fit on a short piece of PVC pipe that is efficient enough to keep up with my EV's. I'll cross them high. Then I'll have coaxials and won't worry about phase plugs anymore.

Dave