Yup, a good ol' dial thermometer will last a long time. The OP and I are using digital ones though, and you really can't leave them in the mash unless you protect them. The plastic container was just a cheap and easy solution. You are right, I usually only lose 3-4 degrees over the course of a mash when it's really cold, but I'm assuming if I can reduce the amount of time the lid is open it's a good thing.
I kind of obsess about trying to make a nice full bodied beer and finding at the end of the mash I'm below
the temp I wanted to be. Over attenuation is a pretty consistent issue for me. I select my yeast strains carefully and do pretty careful control on pitch rates. What's left to monitor but mash temps?