Panels are laser cut from galvanized steel, exterior is powder painted, I've used intake/exhaust fans on same height before but it used to have problems with hot air trapped above GPU's, current setup is performing much better. I might try slats above intake next time I order laser cut parts.
Nice - I wish my laser had enough power to cut steel, but I'm not baller enough to do oxygen assist.
The slats will definitely solve the problem, I tried a similar design before and had exactly the same problem, that's why I suspected why you mounted the fans the way you did. Oh, and you're better off not cutting that spiral pattern out of the metal - not only will it reduce your cut costs dramatically, but I suspect you're loosing 5-10% of your fan performance by having them:https://www.pugetsystems.com/labs/articles/Effects-of-Grill-Patterns-on-Fan-Performance-Noise-107/
All in all I think you did a great job on your build - definitely one of the most well thought out designs I've seen.