I can confirm that a palm slap never fails to really upset a motorist.
but, I have recently purchase myself a Airzound air horn and I am finding this to be my best purchase in years.
You just give them a good blast with this and not only does it surprise the hell out of them and draw attention to their actions by everyone around, but you then feel you have had your revenge so you are not left feeling angry and frustrated
I also give it a blast when any car is parked on the cycle path etc. so I feel I have annoyed them as they annoyed me and again I am left feeling calm rather than angry......
BTW - I have always thought it would be great to make a device something like an inkjet printer head which if a car passes too close it can paint a message down the side of their car as it passes