Compact Low Friction Spool Holder

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 w