Another variation on make-your own: I placed an old car tire on a piece of plywood, filled it with ready-mix concrete, and set into that a galvanized eye bolt with oversized washer fixed in place with a pair of nuts so as to place the washer well down into the concrete. After a few days of curing, it was easy to roll the concrete-filled tire the rest of the way to the shore and up a 2x8 onto the surface of the raft. The raft was used to float the mooring out to the proper depth, where it was rolled over the side (with chain on it, of course.
After a period of use, we decided it was out too far. I hung a chain winch under the raft, connected the cable down a ways on the chain, and used the raft to raise the mooring up enough off the bottom to float it inward to its present location, where it hasn't budged since.
Green's Basis is quiet water. It shouldn't take much to anchor a float securely there. If you make one the way I did, perhaps just an old trailer tire would be adequate.
To approximate the amount of concrete mix you'll need, measure the inside diameter of the tire and the height of the concrete fill. Get volume as
V=3.14*(D^2)/4
and (assuming you measured in inches) divide by 1728 to get cubic feet. The rubber tire will take up some space, so the volume calcuated will be a bit oversized. An 80-lb bag of dry ready-mix concrete will give you just over 1/2 cuft of poured concrete, per this Quickrete website calculator:
http://www.quikrete.com/calculator/main.asp\
The l density of concrete varies, but you can use anything from 130-145 lb/cuft and you'll be close enough for this project.