Just picked this bad boy up from Home Depot:
Perfect fit and sturdy. I just need to cut a notch in the lower shelf for the ball valve to stick through. There's even enough space to get a mash paddle through to stir it.
Going to give it a test run this weekend. If all goes according to plan, I'm going to buy another one. The first will be for HLT->MT and the second for BK -> carboy, with a March pump mounted to take the mash from MT to BK. I may even drill a few holes and siamese the two together.
It's rated for 300 lbs and has lockable wheels.