Old follow up, but I ended up making a "stone" out of a polyform boat fender/buoy. You can get them free if you are around the coast, because they tend to melt in the sun making them un-usable for nice boats after a few years of use.
Pretty cheap on ebay too. They have all sizes from 15" to retarted big.
I cut a small 2 sided flap in the top to pour the concrete (I actually used "precision grout" I think it is heaver than concrete) drilled about 50 holes in the fender for drainage and filled up half way with concrete, added about 80lbs of chain and then topped off with concrete. The outer material makes it easier to pickup (good or bad) and it took about 1 hour to make.
Small ones may be cool to throw, due to the fact that they are very grippy.
I think I have the A-2, second smallest. New it is 32$polyform
It shows 8.2 gallons of space. Concrete is about 18lbs/gallon i think, so with the chain it is about 225lbs.
Pretty cheap way to get a good implement.
The picture is a big ass one, not mine.