If simple and cheap are the consideration then a floating thermometer might work.
I have been using a floating brewing thermometer in my aquarium for years.
They are extra large, maybe 8" long, easy to read, not like the little ones sold in pet stores.
It is glass, so you need to be careful and I only use one because it was laying around, haven't made beer in 15 years or so.
They range from $10-20, not sure of the accuracy of them, would have to check them to be sure.
Just a thought.
