But aren the holes in the backs of the buttons where the switch and LED go in round? You'd have to put your piece of milky plastic at the end of this *round* hole.

I actually tried this, believe it or not.

The problem is that the little holes in the backs of the buttons aren't a simple shape. They are a complex 3D shape that integrates with the switch assembly.

The best that I could do was to use a candle to heat up the white plastic, then "poke" a blunt instrument into the plastic to make it roughly top-of-an-LED-shape. Then the plastic would cool down and I could cut these little white "pimples" off of the plastic. I tried making them have little white "tails" that would scatter the light sideways off to the rest of the button face, but because of the way the buttons integrated with the switch assemblies, I couldn't make this tail large enough or wide enough to scatter the light properly behind the button. In the end, it simply looked like a slightly dimmer LED without much more scattering. And assembling this stuff was a complete pain in the ass, the little white bits would never stay in the buttons properly. I probably could have shaped the plastic more accurately by pushing it against the buttons themselves, but I didn't want to endanger the buttons with heat since I did not have replacements for them.

Then I tried using some translucent white nail polish (my wife happened to have some) on the back side of the buttons. This scattered the light a little bit, but not much, because the layer of nail polish on the back of the buttons was so thin. In order to get the light to properly scatter, the entire buttons would need to be this milky plastic color instead of just a thin layer behind them.

The reason the plastic sheet works for the knob (and it only marginally works, by the way) is because there's three LEDs there.