I just printed and assembled The Rack, and it looks really nice. The latest version of Cura seems to have trouble adding .3mf files (they start floating somewhere off-bed and need to be moved back to 0,0,0), but otherwise they all printed fine.

Some minor tips I can share:

Rails

• Rotate the rail so it lays flat on a face with all the screw holes. (The .3mf imports into Cura at an angle.)
• I printed with no supports, and the holes on each end came out fine.
• If your slicer thinks the 200mm rail is too wide to print on your bed, try changing Build Plate Adhesion from Skirt to None. Cura defaults to draw an outline around the model, and that’s what was too wide for my Ender 3’s 220mm bed. Or you can turn the rails 45 degrees, and then also change your Top/Bottom Line Directions to [90,180] and Infill Line Directions to [45,135] so all of the slicing structures are turned the same amount. (and remember to reset these settings afterward.)

Sides

• I printed two sides in the same orientation, but if you print on a textured bed or just prefer the look of the bottom layer from your printer, you may want to flip one over so you can assemble with matching outer faces.

Washers

• I had to print with a brim to get them to stick to my bed. Since they end up between boards and rails, I wasn’t too precise with removing the brims.

Assembly

• I used M5x16 bolts to attach the sides to the rails, and 10mm is probably plenty. The bolts cut their own threads and seem to hold well.
• I used the M3x6 bolts that came with the boards and printed washers to mount boards on top of The Rack. These also cut their own threads.
• When I attached the Motherboard to the underside, I couldn’t fit any of my Micro USB cables in the hole in the side of the Rack. I printed another set of washers at 150% Z (3mm height) and cut some longer M3 bolts, and that aligned the port much closer to the center of the hole in the side. (The included bolts may have worked, but I had already started cutting down some M3 bolts to 8mm or so.)