The wood flooring in the corner in which I have my sub is slightly uneven and therefore the sub itself isn't sitting well on all four legs.
I know they have the overpriced pads that you can put under a sub but I'm wondering if something more inexpensive like a square of carpet or a square of an anti-fatigue mat would work just as well? Right now I have a small folded piece of paper under one foot as a temporary solution but I'd like something more permanent. I feel like I might be missing some easy answer here.
A piece of carpet would do fine. You can find foam mattress toppers from Walmart for around $10 that would work as well. There's enough material in these to make multiple pads, or double up if you want it.
If you have a downfiring sub, these soft mats have the added benefit of absorbing distortion, whereas hard flooring would reflect it making it more audible.
Right now I have a small folded piece of paper under one foot as a temporary solution but I'd like something more permanent.
I use a folded piece of paper napkin to take up the uneven floor wobble of one of our subwoofers and if one stops looking at the piece of paper doing stabilization duty, immediately the fix becomes permanent. Well, it does in our case.