Reply to very old posts, but found this thread through a search and thought this would help. For Fox 40 fans, here's an attachment for your whistle:
Basketball Possession Switch - Fits in Whistle: Ump-Attire.com
For a "built-in" version, try this:
PC 50 Whistle
And yes, I agree learning to keep it in your head is best, but I'm great and keeping balls/strikes in my head in baseball but I still use a clicker.