I've mentioned this before, but my first CoLo server was from a friend that built servers for a good sized hosting company back in the late 90s. He built it from spare parts but had no case for it. He hung the mother board against a dry erase board from the mounting holes with zip ties. He had a PSU sitting on his workbench with cables feeding up to the mobo. IIRC, the hard drive was suspended on the wall as well. In that vein, anything can be a "rack" if you squint at it hard enough and maybe tilt your head a bit.