How about this for an economical heat source. Use a 50 or 100 watt incandescent light bulb inside a coffee can (to block the light since you need to dry in the dark) inside the box. Or, a small box or galvanized steel duct with the bulb inside it outside of the dryer's intake and a light trap connecting it to the dryer. You could control the internal temperature with a ductstat inline thermostat (available at Home Depot or Menards) hooked up to the bulb.

Personally I think this idea rocks, Simple and 4 days in enough for me to dry.

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Semper in excretia, solon profundum variat!