It is not exactly aligned with your hardware, but might generate some ideas.
Below is a post with 5 GPUs (2 PSUs) in one case: Corsair Obsidian 1000D (now you might consider the 9000) using 3 on mobo and 2 mounted (via extenders) in case where the liquid cooler pump is supposed to be. The are spacers between the extended GPUs to help circulation. This setup has been running continuously on AI models for about a year. I note there are other big cases (e.g., Thermaltake Core W200), but this is the one I have experience using.
WRX90E-SAGE SE with more than 4 GPUs
The final build uses 2 extenders: Lian Li Premium PCI-E 16X 4.0 Black Extender Riser Cable 900mm - Black Color - PW-PCIV-4-90X ] (Amazon.com)
P.S. I am in awe of this build: WRX90e won’t boot with 6 GPUs - #45 by Devinkb