We got a play yard recently, and although quite stable (esp. compared to other plastic products), it still moved and wobbled a bit more than we liked in an open (i.e., non-loop) configuration. So I quickly made these clip-on feet, after testing a couple of alternative designs. Fence now stays put!
The clip-on foot STL is for the North States metal Superyard. I'm attaching the OpenSCAD source (which is a bit(?) messy), since it could be useful for similar configurations: if your fence has a rectangular bottom rail, with a gap between the floor and the rail, then it will be very easy to modify the dimensions (plus overall clip length, etc, if you have a printer with a bigger bed). Other rail shapes rail or fence types will likely require non-trivial modifications. Since this was a one-off design done quickly, I don't have much interest in cleaning it up for Customizer.