I got this one, made by Little Havana. It sits nicely atop my cigars in the humidor, and correlates with my other hygrometers.

The digital Don Salvatore I have is a similar price, is square (maybe attach it to the lid?) and seemed to be "off" for a day or two when I first got it. It's now happier.

I also noticed that Walmart has small, square hygrometers for under ten bucks, which might be OK for a coolidor. (I also bought my Coleman cooler there for under $10).

I can't vouge for the reliability or accuracy of any of these, but right now all 3 of my hygrometers (2 digital and one analog) are in sync, so I'm done.