I've been considering knitting my own mask(s), but haven't been able to find anything on whether it would actually work - like, would it be too porous to do anything? (Plan was 1-1 ribbing wide enough to not stretch, so the layers would kinda double-up on their own)

Jeremy Howard has done a ton of research. Depending on the material, it's around 50% effective, which is incredible given the current situation.

https://twitter.com/jeremyphoward/status/1242894378441506816

That says people are doing it because the government forced them to, not that it is easy or that they are doing it right. In fact the only tip seems to be to make it out of cotton so you can wash it.

And for once I think our government was right in forcing everyone to wear a mask. As it turns out, it is not really hard to comply with something rather drastic like this & the whole nation pretty much equiped itself with hand made masks in a couple days.

I went out today in the second biggest city here in Czech Republic and you can see most people with self made cloth masks, a couple with simple single use disposable masks and just a single person with something looking like a respirator. And zero people without a mask.