Made this for my Ender 3 but you can slide this foot into any 4040 extrusions.
I modeled this in tinkercad and screwed with the tolerances because, at the time, my printer wasn't terribly accurate dimensionally.
Here's the tinkercad link if you want to mess with it.
Print one and see if it fits before printing 4 of them.
This uses foam practice golf balls. You can get them much cheaper than squash balls and they come in a bag so you can share the leftovers with your friends or throw them at your cat if you don't have friends.
you will likely need to clear the supports out of the t-nut channels with some sandpaper. I tried printing this on it's side but I didnt like how the cup came out. Experiment with it, go crazy.