Like many, we've got a bajillion of these acrylic 3 panel shields left over from pandemic things that I've repurposed in various ways. These feet help keep these acrylic panels upright. I don't have the typical standard width measurement in front of me, but the gap is 4mm and they fit nice and snug on the ones I have. 

I included several versions that I tried out. My shelf for one project has a gap between the metal frame of the shelf and the shelf itself, so the small half size ones with a little stabilizer that goes in the gap kept the acrylic panel nice and sturdy and aligned.