Based on Joedaddy's post and a Youtube review, I bought the Heathrow mixer (Ebay, open box, $70 w/shipping). Got it today, and I am super impressed at how easy it is to use and how well it works. Just press the paint container on top of the rubber pad until the shaker starts running. Lift it off the pad to stop.
I was using a budget battery powered rotary mixing tool, which works OK, but always requires cleaning and has the potential for making a mess. The Heathrow works on a closed bottle, so no chance of a mess. Pricey but boy, it works.