I have a whole bunch of the steel shelving units sold at the big box home improvement centers under the brand name Edsal. Some of these I have configured as half-height, and they occasionally get moved around.
Unfortunately I also have a painted basement floor. So that made for a lot of scratched up paint. Having recently re-painted the floor, I made these feet to protect it. I also added felt to the bottoms of the feet with hot glue for extra cushioning.
Just this evening we had to slide around a partially loaded unit to get behind it - and no marks on my floor!
The included OpenSCAD file allows you to enter some customized values if you want the feet wider or narrower, or if the shelf uprights are not exactly the same size as mine.