They are called water wigglers
OMG, who came up with that thing?
Anyway, easiest recipe for a water wiggler I've come across so far: take a zip-loc baggie (quart-sized works great). Put about 2 inches of water in the bottom of it (about the thickness of an average egg). Put a straw in at one end of the zipper and zip it closed. Suck out the air through the straw and zip it the rest of the way shut. Scotch-tape your probe to about the center of the bag, roll the bag and water around it making a cylinder, secure it in that shape with another piece of tape. I usually just lay it on top of my eggs. Almost-free and accurate, two of my favorite things!
