I find that doing them on any hard surface generates callouses, which split, especially in places with wintery climates. So I got a 3mm thick foam mat (about 3' wide by 1.5' tall). It's available Walmart, though I can't seem to find it online. I use the back side so that I'm not distracted by patterning when trying to position my two hands. I also place it on a rectilinear tiled floors, and I imagine where the lines are if the matt was invisible.
Since split callouses are a showstopper in terms of knuckle pushups, I use Dermal Therapy lotion, with 15% urea for hand, elbow, and knee.
P.S. Regarding the comment about buckling wrists, that goes away pretty quickly. It might be an issue for those completely new to knuckle pushups, in which case, moderation should be the key. Once you condition yourself to keep your wrist straight, it doesn't take any energy to do it. Your limb muscles will fatigue and fail and you won't even notice exertion on your wrists. I would say that risk of injury is almost zero, much much lower than hitting a bag, and even much lower than shadow boxing. Knuckle pushups are for people who want to be gentle on their bodies and old geezers (like me) who don't like hyper-extending the wrists. Mind you, you can superset the pushups by switching to normal non-knuckle pushups after you run out of steam on knuckle pushups. The former is easier because there's less travel. A good routine to try is superset with hand close together (less than a foot for me), then wide, then normal. That's in order of decreasing difficulty for me.