I just got my Filco tenkeyless with mx browns - chose it based on all the good info on this site - thanks guys.
Anyway, I've been experimenting with some of the o-ring related mods, so I'm sharing some of my ideas and experiences.
O-Rings
I'm in the UK, so I've been trying to use what's available locally. The best fitting o-rings I found were a 0.208"(id) x 0.07"(thick) - these are a standard imperial size (at least in the UK) so they should be available from most engineering suppliers. For comparison - that's equivalent to 5.3mm x 1.78mm. I paid £5 for 100. They fit perfectly - no stretching required to fit them on the keycap stems, but they're just tight enough to not fall off.
The ones I got are a hard nitrile rubber - I'm guessing they're about 70 hardness which is very hard - I think it's about the same as car tyres. Anyway, I'm pretty happy with them - they definitely improve the keyboard (according to my preferences.) My experience is the same as others have reported - it gets rid of the 'clackiness' when you bottom out the keys. I think it's also changed the way I type slightly - if the bottom of the key stroke is a little softer (and a little higher) it seems to make you hit the keys more gently. Perhaps it's all psychological, but that's how it seems. All in all a worthwhile mod.
Felt
I couldn't find any silicone o-rings of the right size, so thought I'd experiment with some other things. One of the ideas I had was to use felt to make EK style landing pads. Felt is an obvious alternative to rubber type materials - it's used extensively in musical instruments for percussive damping, and it's used all over the place in industry and automotive applications for sound insulation.
So... I got some regular crafting felt - about 2mm thickness. Basically just punched it using a regular hole punch and hacked it in to squares of 'about the right size' using scissors.
My conclusion: pretty much useless! One layer made almost no difference to either the feel or the sound. Two layers did dampen the feel and the sound, but 4mm thickness starts to impede the keystroke too much and things start to feel mushy.
Other Ideas
I've had a couple more ideas but I haven't done anything with them yet:
(1) Rubber Washers.
As an alternative to o-rings. I can't find this mentioned anywhere, but I'm sure the idea must have occurred to others. An M5 1.5mm rubber washer with an outer diameter of about 10mm would probably be a useful alternative to o-rings. I was on the verge of ordering some of these when I had another idea...
(2) Silicone Mat.
I'm thinking of the type of silicone sheet/mat that they sell for baking/cooking. Never really looked at this stuff, but I'm sure it's about 2mm thick, and it should feel softer than the nitrile rubber. They didn't have any in the local hardware store this afternoon, but I might have a go at this when I can find some.
Another possibility would be the rubber sheet they sell for making gaskets - you can get it in quite small sheets in sizes of 1mm, 1.5mm, 2mm etc but it's harder than silicone. A knife and a hole punch would quickly fashion these in to the required landing pad shape. If/when I do this, I'll report back.
Hope someone finds this useful...