Here's what I used in the Ploomidoor. It's another web site, but they don't sell cigars. I hope that's ok.
http://www.habitatmonitor.com/products.htm#Humid
It's very accurate, but costs a few pennies... But if you have a good sized door, and want to keep it humidified, you can go this route. Or the sea foam with prop glycol and water works good too. Less sexy, but works and is cheap.