Spool Rack for Prusa MK2/x Multi-Material

The Prusa I3 Multi-material upgrade comes with parts and directions to build a very nice and functional 4-spool tray. However, it is intended to be placed in the space behind the Prusa printer, or on a shelf above it. I had neither, so the Prusa rack was only useful for parts - like the 16 608 bearings. I took those bearings and a couple of lengths of 20mm V-slot extrusion, a handful of printed parts and built -- a shelf ! OK, it's a "spool holder rack", but that's just a particular kind of shelf, right?

I liked that Prusa's design used the bearings as surfaces for the spool's rims. I'd been using that type of spool holder for several years and it works very well. I had seen similar designs on thingiverse, and have used this concept for the ones I designed for this rack. I have also liberally borrowed and remixed some parts from several makers here (listed below.)

I designed the frame to take up very little table-space, since that's how much I had available. The rack's