Check out some of the maple syrup evaporators. They use an external float box to maintain sap levels in the pans, it might work for you. www.mapletrader.com and http://www.sugarbush.info are both good places to look.
I don't use a water pan anymore but if you are going to keep a pan full you should try to keep it full with boiling water so you don't create a heat sink.
Making memories.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........