I used to use one of the D cell types, in fact I used it for quite a few years until I got fed up with it taking up space (we for months at a time and need the space in the reefer) and dealing with the batteries.
I got one of the 12v units from the fellow selling them on eBay a couple of years ago. No batteries, no space taken up, clip on the fins and it runs, remove it from the fins and it stops and it does a much better job than the little "sit on the shelves" units.