I wanted a very modular hdd rack that was both simple to print and aesthetically pleasing. I went with the 20x20 ext design so me or anyone else could easily print additional parts like a raspberry pi mount, fans, power supply or USB hub etc. Its super solid even with just 25% infill and Im super happy with how it came out. The tolerances are already worked into the design with all of the parts having .25mm clearance which is fine unless you get warping which mine did on a couple but after a little sanding it all came together fine. For each hardrive you need either two of the rounded extrusions or two of the regular ones, a carriage and 4 couplings. Like most of my designs I have added the .stp file for easy modifications. Enjoy!