Just thought of something!
Rio powerheads used to come supplied with a mind-numbing number of additional parts. I have a small box full of those parts that I end up rummaging through every so often. One of those parts is actually a small plastic ball valve to control output on some of their powerheads. I had horrible luck with TAAM products years ago and this box of parts ended up being the most useful TAAM item I ever bought. I have no idea if the still supply all those extra parts these days, but if you can find someone local who has them they may have a spare ball valve laying around. If you want to PM me your address and a VERY precise diameter, I can look through what I have tonight.