The side panels are Aluminum and the rest i expect to make from steel for cost reduction. I wan the side panels to look nice.

Some comments about other questions:

Whats not shown in detail is the GPU risers are rigid mounted with standoffs. They securely mount the GPU.

There is 2" of clearance between the bottom of the case and the intake. Its raised specifically for the purpose of low intake resistance.

Also, for cooling, there are 4 x 140mm fans which is much more flow area than the 3 standard 120mm fans for most 4U cases. This offers an entire extra fan plus the additional flow capacity of a 140mm fan. It will allow for lower fan speeds and there is a direct flow path between the GPU's and the fans.

Also not shown yet is the brackets to stack the cases on top of eachother.