Simple Filament Spool Dryer

This is a simple spool dryer for 3D printer filament. The only external component used is a humidity meter (hygrometer), and even that can be dispensed with if you so choose. Background As you probably know, certain kinds of 3D printer filament are prone to absorbing moisture from the air. This causes problems when the wet filament goes through the printer hot end and turns to steam, producing bubbles in the extruded plastic. There are many solutions to this problem, but most of them cost a fair amount of money, particularly if you only print the troublesome filament types every once in a while. 95% of my printing is done with PLA, which I never bother to dry, and which has never caused a problem. Once in a while, though, I want to use PETG (for increased strength) or TPU (for flexibility). I don't use them often enough that I wanted to pay for a dedicated dryer, so I came up with this solution. It's basically just a box that uses the heat from the printer bed to dry the filament. I pr