This "Compact Low Friction Spool Holder" is intended to be used with my "Ultimaker2 Rail System"

 https://www.youmagine.com/designs/ultimaker2-rail-system-u2rs

 and is a modification of Roberts well known Low Friction Spool Holder; http://www.youmagine.com/designs/882. The component "608_core2" is unmodified.

It's a bit of a strange choice maybe but I based this thing on 608 bearings (=M8) and a long M6 Hexnut, simply because I had this available (and did not want to use a M8 bolt for weight saving). I added the DSM file so you can easy adjust to other bearing dimms if you neeed to.

One of the 2 end holders will go over the head of the M6 hexnut completely, so you can mount 2 spoolholders close together on the rail system (2 spools in one bracket inbetween them) I made the 50mm spool adaptor for UM & ColorFabb spools, with the DSM file you can easily modify it to any size you need. This component could also be a good base to make a "loose fillament adaptor" I'll may make one later if ever I need one...