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.
Laser is not mine of course it's an industrial machine. I've seen that study about grill patterns and the one I've chosen is a trade off between performance and cut costs. I would prefer wire grills but I haven't found one for that fan size at reasonable cost, yet I want to have something in front of them.