The PVC cement on my slosh pipe is curing right now. I built a 4 in diameter, 8 foot version using the article Ross posted. I had considered going for a 3" diameter to make grip easier, but I decided to maximize imbalance instead.
I did a bit of math (okay, a lot of math), and figured out how a 3 inch pipe would compare to a 4 inch pipe, not only in terms of total weight, but in imbalance (or torque). I'll only bore someone with the details if they ask, but as few results were interesting.
First, a half-filled pipe provides the maximum imbalance. It may not be the heaviest, but if you're looking for the most sideways torque, half full is best.
Next for half-filled pipes, to get the same imbalance as an 8-foot long, 4-inch pipe like mine, a 3-inch diameter pipe would have to be 10 feet 8 inches long. The slight change in diameter really affects the volume.
And going to a 10-foot long, 4 inch pipe (from 8 feet) makes the imbalance a lot higher, from 41 ft-lbs to 65 ft-lbs. I tried a pipe sized like mine last week, and this will be plenty, thank you.
So, there's the skinny on slosh pipes. I'm looking forward to playing with mine.