I just got this off Amazon, thought I'd pass along since it's a decent price with free shipping (super saver rules apply):
"Thermaltake Mobile Fan II"
Amazon.com: Thermaltake Mobile Fan II External USB Cooling Fan - Us: Electronics
It's USB powered, so the draw is only 5v, 0.5A max. It comes with a 6 foot USB cable, the spring wound type. The cable is very flat, so it would slip under the lid seal quite nicely.
It gets a little loud if you crank it up, but at the lower end it's quiet and moves enough air. Inside the freezer, I doubt you would hear it.
Since it's USB, you can plug it into any USB wall plug, like an iPhone charger cube. I went with USB fans, because they're cheap, low power, USB is a standard, no wiring.
The other nice thing is it's all assembled with screws, so it's easy to take apart. I'm not sure if it has enough power to spin heavy magnets, but since you can strip it down with a screwdriver, it's a cheap variable speed stirplate option.