This is a spoolholder that runs on ball bearings for Materials like TPU that require the least possible friction. It fits in the original Spoolholder arm from the MK4. For the assembly you need an m8x80mm bolt and (of course) 2 ball bearings. To make it as cheap as possible i used 608 bearings that are used in skateboards and are widely available for very little money. you put in one bearing from one side and then the middle tube, then the other bearing. This is so when you tighten the bolt there is no load on the bearings. 

I cut a fillet in the splitting part, but not till the end, so the splitting part actually opens up when tightening the screw. The fillet could also be printed, if added in the cad. Or just heat up the bolt.

The assembly works well and was tested on many prints with tpu, it reduces the friction to almost 0. Works with plastic and cardboard, makes no difference.

I printed the part with PETG with a .6 nozzle and basically its only perimeters, no infill. 
Im sure it will work fine with any other Material, its not at all critical at any point.