I have a number of industrial racks I use for servers and storage. They’re a fairly standard design: angle iron and particle board. Interestingly, some came with no feet (which would be fine on concrete, but I have actual tile floors). Some came with only four feet. Some came with eight feet, the other four serving as caps on top of the angle-iron uprights.
When I first got a 3-D printer, I was excited to discover that somebody had already uploaded a design for feet to Thingiverse. The design was for much larger shelves, but it worked—it was, perhaps, the first actually functional print I made.
I assembled another shelving unit today, and realized that I didn’t need to use the oversize design as I now had the software and skillz to do my own appropriately-sized design.
FreeCAD and .stl files are on Thingiverse.