Our playpen originally came with rubber rollers that fit on the bottom to protect the floor. The dogs tried to eat the small rubber rollers, and the frequently would pop off and be lost to the depths of the Roomba. 

These are replacement stands that lift the fence/pen 3mm off the floor (~1/10 inch) and support it from tipping. They slide on.

Designed to fit the pet play pen from HomeDepot, but likely fit any similar metal grate. Fits 3mm to 6mm wall/grate width. Slides on, slides off. 

I printed mine in Polymaker PETG on a X1C.