I designed these to work with a Prusa i3 Bear Full Upgrade (https://www.thingiverse.com/thing:2808408) based printer that I built. The Bear is a wonderful design, but it is very rigid which means with my old school step-sticks it is kinda loud.
These feet use the silicone injection mold making method shown in ScottyMakesStuff's video at https://www.youtube.com/watch?v=0iZ8L1HDubY
I found his recipe worked really well and it was easy to do with readily available silicon from the hardware store. I did sample tests and varied the mix till I got one that was appropriately firm when set.
There are two mold halves that assemble with M3 screws and nuts and a plate that gets inserted inside the mold. It only fits in one way, so it is pretty straightforward. When the silicone is injected, it mates mechanically with the plate. After it sets, you can remove the resulting part from the mold and you have a nice little foot you can place under the feet of the Bear (it is design to sit in there snugly). In the picture, the black part is the silicone and the red part is PLA.