RustySpringfield
I'm all over it!
Got one of these: http://www.livefoods.co.uk/product_info.php?cPath=51&products_id=219if the ambien temp of the air around the water is say 70-75 an inch deep waterdish will evap in 10 days. Or. Many pet stores sell inverted bottle bowls for hermit crabs with a screen so nothing can crawl into the bottle that will gaurantee the water lasts, the only issue there is if your kid decides to poop in it. They are pretty cheap too. about $6 US.
I'll only use it in conjunction with another water bowl, and only whilst I am away on holiday, but I think it's a nifty little thing which solves my problem perfectly.