Its extruded aluminum. Comes in various forms . . . 8020.net or Makerbeam is available on Amazon.
You can also get fully made cases for Spotswood.
http://spotswoodcomputercases.com/wp/
anyone knows what is that metal bar that is used to mount the gpus?
am I right to say the gpus are "clamped" on to that extruded aluminum?
jwarren81 got it right - it's 8020.net extrusion, they're my preferred building material - super easy to reconfigure, and when I originally designed this particular case, I wanted them to be strong and stackable and they're definitely that. I ended up putting them on 5 shelf bakers racks because it just makes them trivial to pull out and work on when needed.
Spotswood uses another type of extrusion for his cases - I believe it's probably Misumi but I'm not sure. It's a much lighter/smaller type then I use, but he's also not really designing for the same use case. I've checked out a couple of his cases, and I think he does a really good job on them, plus he has some custom mods he does in terms of screw holes, etc that I generally am too lazy to do or order.
As far as the clamping - yes, the GPU's are all clamped into position - no major reason for this other than I wanted them rock solid so we could easily grab a unit and move it between locations, or even ship it fully built if needed. You can drop them (and I have), and really they're virtually indestructible. I sold the ones I had in my prototyping mine to another forum member, and the airflow from them as so good, he put his open air frames on the exhaust side of my frames, and it stopped them from overheating - pretty cool.
I also have a 7 card design (although I only built one), and I was thinking about making a 12 or 14 card version that would have the same footprint, but haven't gotten around to changing things yet.
For the most part I prefer the 6 card systems, but a 12 card box would be pretty awesome.