These help hold your buttons in place, prevent them from rotating, and limit how far they can protrude (so they match how far the A/B buttons can stick out). They are about as close as you can get to an exact match to the A/B button wells. Just position them and put a couple tiny dabs of hot glue on either side to hold it in place, and that's it!
These are made for the GB and NES buttons, although the NES rubber mats are a little finicky with them I've found (if you get a GB A/B button rubber pad then the NES buttons work great!). They should work with official SNES buttons as well, provided that you shave off the extra peg on them.
I print them separate from each other so it will work with anyone's button placement (since these holes are hand-drilled and there will be some variation between builds). Depending on where your screen is placed you may have to shave off a bit from the top of the one for the X button to make it fit (or move your screen up just a bit).
They are printed in a polycarbonate composite material from Push Plastic that is very strong (it's what I used for the screw brackets), on a Lulzbot Mini. I'm selling them 4 at a time since they are tiny, so you'll have a couple extra in case you manage to break one or something.