The spikey points get printed facing up, but rather obviously face down into the carpet in use.

Ours was printed on a MakerGear M2 with cheap black PLA at 20% fill, which is more than enough to produce a very solid stop. A few days after posting this, one of my students gave this a real stress test: attempting to force a stopped door, the stop didn't move but a slight dent was made in the plastic. The slight dent is totally harmless, but no doubt a little heavier fill would have prevented even that....