I needed a way to fit four spinning disks in a space designed for a GPU to make a small server, so I designed a couple brackets to hold them in that shape and mount to PCIe slots with the I/O all facing in. It's very clean and stable. There isn't a lot of room for ventilation, but I haven't noticed any performance issues so far.

All parts were printed in PETG 0.20 with no supports and no special settings

for the four drive configuration of the server in the photos
1x bracket
1x back
2x side
2x top
or...
replace sides and tops with two short-end for a two drive configuration
or...
add two back-to-back and two more sides for an eight drive configuration
or...
replace the back and bracket with two more top pieces to have a stand alone set
or...
use four pieces mounted to a single drive on the same side to use as feet