After wrestling with my (god-forsaken) camera's macro and manual focus "features", i managed to get some better shots of the washer.
Note how the washer "folds up on itself". This is a good thing. The washer is slightly too small for the bottom stem, so it 'folds up' when you stretch it to fit around the bottom stem. Its good because this is what causes the reduction in the smacking noise. (It also happened to be the largest size washer they had in the store which looked like it might fit). If the washer doesnt fold up like that, then the reduction in smacking noise doesnt happen.
inside diameter: 4/16" (or about 6.5 millimeters)
outside diameter: 1/2" (or about 13 millimeters)
thickness: 1/16" (or about 2 millimeters)
Measured that with a micrometer
So ya, you can try reproducing this on your keyboard, or maybe the O-rings mentioned above is a better idea. Basically it will take some trial and error, but the bottom line is that washers or o-rings can solve this problem quite well if you are willing to play around a bit and try different things with them.