Universal spool holder for any popular spool diameter 51-60mm, easiest to use, any caps, any complications necessary for mounting spool, just plug&go. Before this I made some other holders, also saw a lot of kinds, but in my opinion this is the best :)
I created it for mounting on the side of my X3D printer, but here I added also “flat” version for mounting on any flat surface.
Holder has two ball bearings and a kind of brake to avoid uncontrolled unroll of filament. During printing it generates quiet sound “tik-tik-tik”, but I believe it shouldn't be annoying, it is really quiet sound. Optionally you can don’t use this part (and have totally unbraked holder) or put felt ring between movable and immovable parts. But in my opinion brake made as it is described below is better.

Material list:
1) printed parts (please note that part A has two options)
2) 2x ball bearing 6905 (25x42x9mm)
3) Some springs. You can use 1-3 springs for any inset depending on the hardness of them. In my project I used 4x2 springs 14x7.5x0.6mm and it is ok.
4) few mm of springy wire or stripe (part of small watch spring is ideal). I used springy wire 0.5mm, part of an old spring found between my “things that can be useful”
5) 4x screw M3x20 or woodscrew 3x20, with countersunk head (I’m not sure if it is correct word, but I believe it is obvious, what kind of screw here must be used)

Some mounting tips:

• you can easy mount insets with springs by using small metal ruler or something like this. Just put springs into the holes, push them with the ruler, put together into the body and remove ruler.
• springy wire/stripe you can mount in proper place (see photo) using glue or just warm it and mount by melting plastic around of it (method used by me).