OB is another idea, but I don't think it should be closer than about 30' to the basket. For one, that isn't as much fair as it is lucky with the way shots roll off the baskets. And if it is much closer than that, you next putt is from a gimme distance.
Thinking of that made me rethink my moat idea. It would be way cooler to have the moat surrounding the green at the 30' mark.