Unfortunatelly my last slot is PCI. I know a PCIe NIC would fit, and I do have one, but the x1 slot is up, not "down there"

nice!

What you can do is install the hard drive in the 5 1/4" drive bays, which will free the bottom of the case. I'll take some measurements for you though.

I considered that, too. Unfortunately there would lie the unused PSU cables. Unless I'd do some cuts in the side and hide the cables beneath the MB tray.
But that's a bit too much for my skills

Yeah, things do get tight in the 5 1/4 bays because of the power supply cables.

By the way, I just checked the clearance between the bottom of the hard drive and the last PCI slot and there is no space for a full height PCI card. The hard drive pretty much blocks access to the PCI slot, but there is less than 1 1/2 inch distance between the hard drive and the PCI slot.