While @Elecrow is taking their time with the order (still in production after 10 days), all the buttons arrived for testing. I really like the idea of laser-cut button caps, but if that doesn't work, I want to have some other options, so testing continues. The buttons look like this:

From the left: the original 6x6x6 button I used in PewPew, the silicon switch I used in µGame, those horrible 4x4 switches that dig into your fingers, low-profile 6x6x5.5 switch, and ALPS SKRAAME010.

In the back you can see one of the cases I experimented with, with two layers, bottom 2.5mm and top 1mm.

Some immediate conclusions: the metal one will need keycaps one way or the other. The two low-profile buttons might work without (especially the ALPS, that has silicone cap), but seem a bit narrow. The ALPS button is minimally higher than then 2.5mm required to fit under the top layer.

I will need to wait for the prototype to be able to say anything more.

In other news, I'm also taking a look at the competition: in this case, one of the oldest homebrew consoles out there, the Arduboy:

Some interesting things to note here: they have brass nuts sunken in the plastic for the bolts — that probably makes it impossible to crack like it did for me. Buttons use plastic caps, but underneath are just simple metal domes, glued to the PCB with a sticker, not even soldered. The USB port is interesting as well, because it's sunk into the PCB's cutout.

The other side is mostly a super-thin LiPo battery, with horrible soldering, and a piezo stuck under it. Kapton tape guards against shorts to the metal back.