Ouija boards don't work unless the participants can see.

Let's say spirits exist. Let's say something about an ouija board enables communication with them. What is the something? is it the wood? the ink? The patter of the wood, or ink?

I say, none of the above. The key element is the person involved. Think about mediums, people who claim to channel spirits. All an ouija board would be, then, would be a suitable context for someone to desire or will themselves to want to contact spirits. The mechanism is the person themselves. The fact that you have people "making" ouija boards with pen and paper in this thread, or of "alternatives" to an ouija board like the somewhat hilarious practice of gyromancy suggest this is the case.

Further, occult writers have long advocated and sought after altered states, whether by meditation, tantric sex, hypnosis, self-or-otherwise, and of course, pharmaceuticals. And in regards to that last, very popular option, people on certain categories of drugs report encountering apparently autonomous beings

Now, of course, this doesn't prove spirits. In fact, doesn't prove any of the experiences listed above aren't internal in origin. But evidence? To go back to my first point, if your operating assumption is that hypothetical spirits may just really dig certain arrangements of letters on wood, I think you're limiting your investigation. More to the point, if our capacity to investigate this phenomena is limited to subjective interactions of individuals, then you are able to dismiss this sort of thing as being without "evidence", mainly because your parameters for evidence, in fact, dismisses all of the datums we can even work with in these case.

This is, I suppose, practical enough, as long as you're okay assuming lots and lots of people are just unreliable and/or liars. Mind you, we don't really know how people work either ( 1 2 ) but don't let that stop you.