Spool holder that mounts on top of Mendel901) Print out one holder.stl and one holder_mirrored.stl 2) Attach to Mendel90 with 6 M4 x 20mm screws and nuts or tap the sheets and use M4 x 12mm (have not tried this as my attempts tapping acrylic varied. 3) Use 380mm long M8 steel or threaded rod to fix the spools.On my Mendel90 (mendel variant, 6mm acrylic) it can hold up to two 2.3kg, 300mm diam. spools without problems.One could design spool center pieces which hold 608 bearings but I suspect that it would be too time-consuming to fit the centers to new spool.Another issue with this design is concerned with PSU mounting- PSU has to be mounted using some washers as spacers.