polypropylene balls

In my setup I usually cover my tank with saran wrap, very little water evaporates, even on long meals and hotter temps, though I do make sure to top it off every once in a while.

I just got lids for my setup, before that it was open. It lost one or two inches or so of water a day when cooking at 131F so I just tried to refill it before I went to bed at night. More was lost at higher temps.

I agree that Saran wrap should work real well and I've also heard of people using ping-pong balls. I guess the assumption is that the cover doesn't have to be food grade since the food is "protected" in plastic. As I always say about the plastic issue, I'm not an expert in plastic so make your own decisions if it's safe :-)

I have been using a Presto deep fryer, it was shallow, lots of evaporation, lid would not fit down well over the temp probe wire, etc. Before Christmas Amazon had a Butterball Turkey deep fryer for $65 with free shipping just as I received a royalty check for $43.
It's now about 30 hours into a corned beef brisket. The evaporation has been much less with the full cover, when I open it to check the condensate runs off into little pockets in the corners. I'll get an ear syringe to clear those out tomorrow. Over all I really like this over the Presto, the water is deeper allowing larger pieces and it has a drain valve to make it easier to empty and clean.
I plan to do a few more experiments, take some pictures and do a crude review of the set up.

GeneK

I use a slab of polystyrene that I cut to fit my water bath. It floats on the water and helps keep things submerged as well as acting as an insulator.