Ok, I got it now -
You can make one at home: take a ziplock sandwich bag and put some water in it. Not too much, but then fold it over and slide your thermometer into the fold - it measures the temp of the fluid (mimics the center of the egg) instead of ambient air temp. Then you adjust your thermostat to keep the center of the wiggler at 100F or whatever...
You might have to play with it a bit to get the water amount just right and then you could use a bit of masking tape to hold it in the folded position.
