Check
this out, it's a review of that cpufx crap water cooler, but it shows the detail of the 12v pump design in detail (also based around a 12v fan motor)
may be able to point you in the right direction.
I would personally, based on my own experience with modding pumps, use a flat disk between 1.5-2'' in diameter with small vanes on it, on the order of 1/16'' high, I'll draw a diagram later.
(basically similar to how the impellers on the high head iwaki's are set up)