I go with either:
Pat the top of the helmet as in "topper ahead", or, while holding the hand level over the ground (at my side), lift my hand up 'n down, as in "slow down".
The first one seems like it might be obscure, because I've had some ask me "what are you doing"?, "signalling a cop ahead", "oh, I dont get it..."
But, the second signal, slow down, seems pretty good, I think most riders get that one.
Trouble is, that one does not work on the freeway at 70+ MPH. For that, I throw my arm in the air, straight up. It's unusual enough to get the most attention from the opposite side of the median (and is not just a "friendly wave").
But, that one is obscure, too. It doesnt say "cop ahead" definitely.
You got anything?