This is the same thing that I do.......
I have a 26 watt heatmat under a 5 quart rectangular glass baking dish.
I put about a gallon of water in the dish, this way I can carry it into the bathroom to clean it with out spilling too much.
This heatmat is plugged in all the time to increase evaporation, hense it increases the hudimity in the tank.
I bought the Anchor Hocking dish at WallMart (cheap price)
Harry