The cantilever fan attaches through the back grille;
https://forum.45homelab.com/t/hba-fan-bracket/511
45Drives calims to use it on their enterprise gear, but I have the same doubts as you as to actual effectiveness.
I think the Fan Shroud is supposed to attach to the back of the 120mm fan on the fan wall that blows over the PCI slots. How well that lines up with PCIE cards that are taller than full height, or long, I don’t know. I think these parts were originally designed to cool hot HBAs and NICs, not hideously oversized GPUs.
The fan shroud is available for 3D printing if you think you could modify it to fit your PCIE setup;
https://forum.45homelab.com/t/fan-shroud-design/2428
It is possible with zip ties to mount two 80mm fans over the IO shield, whether that would help with your cooling issues, I’m not sure;
https://forum.45homelab.com/t/6bay-ssd-bracket-kit/3512/4
You might need to adjust your fan profile to have higher RPMs on the front and mid fans serving the PCIE slots, or add a 40mm fan and a duct to push air umder the GPU or whatever and over the SSD(s).