I tried to put together the best design elements of few other spool holders published on this website and created my own universal spool holder for K8400. It can hold spools with hole diameters from 30mm to 62mm. The maximum spool height depends on the hole size - the wider holes allow to fit higher spools. With maximum hole size of 62mm, spool height can reach 120mm.
For this holder I designed my own thread with 45 degree sides that allow printing it clean without support.
The holder is attached to the original mounting piece of K8400. It uses 2 plain 608 bearings and one M8 self-locking nut (all parts of the original K8400's spool holder).
The only piece you need to purchase is M8x120mm bolt. It will replace the shorter (M8x70mm) bolt that came with the printer. New bolt can be fully threaded or, like in my case, may have only 20mm long thread at the end.
Check also my filament ballast that can be used with this holder http://www.thingiverse.com/thing:1887621
UPDATE 25-Dec-2017 Uploaded big-nut_rev2.stl with cutouts that reduce overall weight of the thing.
UPDATE 13-Feb-2018 Created the brake to address smoothness of this holder: https://www.thingiverse.com/thing:2792583
I recommend printing big-bolt.stl and cover.stl files with 0.1 resolution. Other 2 files can be printed with 0.2mm resolution. I used 100% infill, but this is most likely overkill. You can experiment with lower numbers and share with us which value worked for you.
I used 7mm wide brim with all parts except the cover piece. I print all parts directly on the glass and use hairspray as adhesion. It takes several hours to print big-bolt and big-nut and on my 1st try both pieces slightly pilled of the glass, however both were still usable. You may consider gluing down the brim with super glue while printing.
1) Before assembling the holder make sure big-nut piece can be easily put on big-bolt piece, that it rotates freely and can reach the very bottom of the thread. Depending on the quality of your printer and filament it may work right away or require cleaning/sanding.
2) Remove the original spool holder from printer's side panel and disassemble it. You will reuse 2 bearings and 1 self-locking nut from it.
3) Replace M8x70mm bolt with M8x120mm bolt - put it through the mounting piece. Do not attach it to the printer yet. Put the mounting piece on the desk with the bolt thread facing up.
4) Place one bearing on the bolt and place big-bolt piece on top of it with the wider side facing down. The bearing should easily move inside the hole of the big-bolt.
5) Place the tube piece on M8x120 bolt. It will act as a spacer between 2 bearings.
6) Place the second bearing on M8x120 bolt and slide it into big-bolt all the way until it lays on the top of the tube piece.
7) Place self-locking nut on M8x120 bolt and tighten it. Make sure not to over-tighten the nut. Big-bolt should not move along M8x120 bolt, but at the same time should freely rotate.
8) Place the cover piece on the top of big-bolt. Its only purpose is to hide the nut.
9) Attach the holder assembly to the printer. Big-bolt has 4 cutouts that are aligned with the positions of mounting screws.
10) Place the spool on the holder and tighten it with big-nut.