Horizontal mount spool holder is designed to use minimum plastic, uses no non printed components, no bearings, nuts, threaded rods and is incredibly simple.

you need 2 spools with standard size 1.5 inch diameter spindle hole 1) print ring and semi-sphere components and join them into a mushroom shape: http://imgur.com/qRrIrd7 they should fit together snugly without glue... 2) place the spool holder into the hole of a secondary spool you are not using or in another stable surface... http://imgur.com/STOgpxi 3) Place the spool you wish to use on top of the semi-sphere and feed filament into your printer. The weight of the spool is concentrated at a central point with low friction area, as a result it is both held securely and can turn freely. http://imgur.com/MTtB3Oh I have found this approach to be simple and effective, please try it and make improvements if necessary.

Attribution - Creative Commons

spoolholder