Making a keyboard quieter is a quest undertaken by many enthusiasts. Most of the methods for doing this require either complex switch modifications or slapping o-rings on keycaps that ruin the feel. Another option is Zealencio silencing clips. These neat little devices come from ZealPC, the designer of Zealio switches. However, they're not only for Zeal's switches. They'll fit on almost any Cherry-style switch to provide a quieter typing experience.
You can apply a Zealencio to the switches already installed in a keyboard. They clip on top with a hole for the stem and another for the LED. On top of each Zealencio is a thin rubber pad. This pad is double-sided in order to silence the bottom-out noise as well as the return.
Zealencios work best with GMK's Cherry profile keycaps. There's a cross-shaped support on the underside of those caps that makes the stem more sturdy. A lot of caps have this same sort of structure, but Zealencios are just the right height for the GMK support to land on them. That keeps the switch from bottoming out completely, resulting in less noise. The press also doesn't feel as mushy as if you add o-rings to your keycaps.
So, to get the full effect of Zealencios, you need GMK caps or something very similar. However, the other end of each press will be quieter for all keycap types. As the stem returns to the top, it'll hit the bottom of the pad instead of the housing.
The really cool thing about Zealencios is they'll work on almost any switch that uses a Cherry-style housing. You don't have to take the switches out, and it doesn't matter if they're mounted in a plate. They're also translucent plastic, so they won't (entirely) interfere with RGB underlighting if your switches support that.
Zealencios fit over the top half of a switch's housing. They lock in place with a small plastic tab on the left and right side—they tuck in right under the lip that separates the top and bottom halves. It takes some practice to get them locked in place, but you just have to apply a little pressure in the right spot. Once they're attached, Zealencios will stay in place forever unless you decide to pop them off. Again, it takes some practice, but you'll get the hang of it.
ZealPC is taking orders for a new round of Zealencios right now. They'll start shipping out later next month. You can get them in bundles of 70, 100, or 120 for $45, $60, and $70, respectively. So, that's between $0.30 and $0.55 each. That will add substantially to the cost of your board, but they really cut down on the racket.