About 15 years ago I found in an old music store, where, oddly enough, most instruments were not for sale, 3 brand new (don't know how old or who made them) harmonica megaphones. They were made of a thick flexible cardboard with a little holder for the harmonica at the small end. In actual use, I can't tell that it does a lot for the sound, but it sure does look very cool. I plan to make one out of aluminum and see how that works. Sometimes it seems that it's all been tried before.

Give it a try, Chet W.