A foot that stops the door. A more long winded explanation would be:

A plastic version of what goes in shoes, cunningly shaped into a wedge such that it may encourage doors to stay put.

Importantly, one side of this is angled and ends up with a lot of ridges when printed. That makes it great for gripping the carpet.

Print one, mirror it and print another. Don't put in shoe, put under door... (Clearly this is good for 2 doors or just one really stubborn one)