Been repurposing my empty filament spools for yarn and needed somewhere to store them, and make it easier to transfer onto smaller bobbins. All the other “door hooks” I could find sit flush against the door, and wouldn't work with spools, so I designed this.

It holds spools a distance from the door so they can spin freely, despite being designed to hold the weight of yarn, it should be stout enough for a rack of filament depending on what you use for the bar.

Fits a standard indoor hollow core door, about 34mm is what I measured mine at. Holds a bar up to 40mm. I actually printed 4 pipe extensions and an end from Adafruit's “Yet another keyblade” found here: https://www.thingiverse.com/thing:3582947 and it holds up well, but I would definitely choose an old broom handle, dowel, or something else if you plan to store heavier things like filament. 

Also, it prints at 15mm, use the scale function in the slicer on Z to scale up to 20mm or more for added strength. I suggest 4 perimeters and 100% infill to reduce flex at the top hook. I've added the gcode to print on an MK3S/+ sliced with my recommended settings, and is what I used for mine.