Surely someone has thought of this, but I thought I'd post it anyway. I found it a pain to whirlpool by hand after cooling down the wort, so I came up with this. All that goes into it is a crank from a barley crusher with the non-threaded hole drilled out larger (~1/2") and a plastic mash paddle cut short.
The test run (in cleaning water from yesterday's brew) produced a nice whirlpool and a neat pile of sediment at the bottom/center.
I was able to rest the drill (battery) on the keg rim and it's not bad. If it's not rested on anything, it's not easy to hold straight, but not impossible. As for torque, I was surprised myself... it cut through it like butter.