Prusa i3 Plus Spool Holder v2.0

Based on the original spool holders from Prusa. Changed it so they clip tight under the top of the frame. Press them down from the top and push the bottom in place.
Also added support for a 8mm rod. I used a 110mm long rod and combined it with a spool hub to adapt the 50mm spool inner diameter (plenty of those on Thingiverse).
I used a customized version of this one http://www.thingiverse.com/thing:605360 .
Also included the clip on system with the original Prusa i3 top spool holders as I fou