The fan is 40mm x 10mm. A suitable hole is cut at the back of the lid using a hole cutter (not for the faint hearted). Contact adhesive is used to secure the fan housing. The 12 volt fan is run from a 5 volt supply (possibly a power bank that has no low power cutout). A silica gel bag is placed inside the housing ( with enough room to allow for possible expansion). The lid stop is modified from http://www.youmagine.com/designs/sunlu-filament-dryer-lid-stop