We've had folks that hook a small plastic milk bottle to the top of the mast (or tied to the near the peak of the main). You're obviously not going to get the fastest times out of your boat doing that, but I'm not a big racer so my experience comes from that angle. The floatation it provides won't stop the boat from turtling in a high wind, but it sure helps otherwise
One of the key things I've found (we too have a shallow lake) is that you need to get around to the centreboard quickly before the dinghy completely turtles. As soon as that mast gets stuck in the mud, you're generally $#%& out of luck (and I've seen masts broken/cracked by trying to bounce the boat upright after mast embeds itself in the muck).