I liked the idea of a few heated dry box designs that I've seen, and decided to extend the design to include 4 filament spools in a single box that feed into a single tube connected directly to the extruder inlet. This way, I don't need to swap rolls around when I'm changing filament, I just roll one back and feed another one through. Everything stays connected, sealed, and dry. The heater is a 7W reptile heater that is on a smart power bar that turns on when my printer is turned on so it's heating and drying anytime it's printing. See here for original design: https://www.thingiverse.com/thing:2778707/
The 4 tubes leave the box from 6mm cable glands and merge at the filament tube merger, combining into a single tube that feeds to the extruder. The final connection is made with a chunk of Bowden tube (that fits perfectly inside the 1/4" o.d. and 0.17 i.d. polyethylene tubing, held in place by my filament tube guide found here: https://www.thingiverse.com/thing:3696299
The benefit to the Bowden tube chunk is that it essentially seals around the filament as it passes into the extruder, keeping moisture out of the box and eliminating exposed filament during periods of non-use (not sure if "periods of non-use" is even a thing for anyone). It also allows for some expansion and retraction as the extruder moves up and down on the Z-axis, keeping everything nicely aligned and sealed.
This particular thing is the filament tube merger component.
2019-08-31 Version 2: Edited the angles and made a more sweeping internal passage to better align the inlets/outlet in order to reduce friction. Performance improved!
Filament spool rod hanger: https://www.thingiverse.com/thing:3676334