The Windtec works great.
The Claw by Latin Percussion is pretty nifty too. What I like about it is the fact that the umbrella can be angled. I've had situations were there was light rain and even though I had an umbrella, there was just enough wind to sort of "push" the rain up, and under the umbrella.
Doesn't happen all that often, but I like to be prepared for anything when it comes to taping outdoors...