Are you aware of any 19 " rack that could be used if the NVMe base is attached to a RPi5?
Have a look at this: Raspberry Pi Rackmount Kit | 4x Slot | UM-SBC-213 | racknex
They explicitly support PCIe-expansion below the Pi5.
I have one of the smaller 10" racks (sufficient for my use case), and that is why I decided to switch to a different NVME-adapter on the top of the Pi5. I use the NVMe-base for my desktop-testing units.