This is what I did to reduce the upstroke clack on a plate mounted MX switch. I cut some foam I had around into 2 strips and inserted them into the 2 stabilizers. There is not a correct length, width and thickness of the foam because it depends on the type of foam you use. I just did it by trial and error. The reduction is not dramatic but it makes the clack more bearable.
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Interesting.
My experience is that the upstroke noise on Cherry MX can be reduced by injecting some amount of grease into the switch. The effect of the grease is to reduce the velocity of the switch slider, which is pushed by the spring on the upstroke.
I use EliteKeyboards' blue mechlube, which has the right viscosity and the syringe-like dispenser is well suited for the task.
If the grease is too thick, your switch will be slowed down too much. If it's not viscous enough it will not do anything for the noise.
I have also used silicone grease from a hardware store with success.
When you put the right amount of grease, the switch feels slightly heavier, but just a little bit. The upstroke is significantly more silent. Also, the switch makes less noise when you move the key laterally. When you use your hand to brush lightly the keycaps, it makes significantly less noise.
If you want to try this, start by injecting a little bit of grease on the right and left side of the slider. You need to push the slider down and lube the inside of the switch. Add more grease until the effect is OK. It can be a good idea to refrain from adding more until you have tested the keyboard for long enough to be sure you want to add more grease: once it is in, you cannot easily remove it.
Don't use this method on clicky switches like MX blues, it would probably kill the click. It's OK on tactile and linear switches (red, black, brown, clear, ...).
