Tony, try making a blob of 'clear' silicone rubber sealer (Ratshack sell tubes), and then when it's cured try pushing an appropriate sized blob into a button. I've been experimenting with grinding down LEDs to change the focus (marginal results), and for kicks I put a blob on the end of one...and was pleasantly surprised by the diffusion. (The blob is approx 3-4mm thick.) Unfortunately I don't yet have a button kit to try this out with

I suggest starting with a well-cured blob cause you should be able to pull it back out with tweezers, ie non-destructive testing.