A probe for a digital thermometer might work, but not the analog thermometers advertised for this purpose. I bought the latter and installed it and it's not worth the money. First, the thermomter I bought for this is not very accurate which is a major problem with mashing. Second, the analog thermometer's probe interferes with stirring when you dough in or batch sparge. Third, the directions told me where to drill the hole to install the probe. It's too high to measure the water temperature before I dough in which means I can't tell how many degrees I've lost to the cooler itself and from trasfering the water to the cooler. I'm about to pull out my thermometer and plug the hole. Oh, there is one plus......it looks cool! Like a real scientific-like brewer's mash tun (except that it's orange and says "Igloo" on the side).
If you plan to use a digital thermometer and install the probe in the cooler, just make sure you install it so it can measure the temperature of the water before you dough in and that it doesn't interfere with stirring/mixing. Good luck.