Contrary to popular belief, o-rings don't affect key travel
unless they are elastic, in which case they affect key feel. Here is why:
Assuming an o-ring made of an inelastic material (e.g. nylon washers), you would simply end up with key caps that stand a little higher and, more importantly, an a partial mate between the shaft and the key cap, resulting in a gap as shown in the picture. Key travel, however, remains unchanged. During o-ring installation, the shaft bottoms out as usual, but the key slides onto the shaft partially because the o-ring is in the way.
If the o-ring is made of an elastic material such as rubber or silicone, at installation time the o-ring undergoes deformation. If enough force is applied, the cap will slide further onto the shaft and the gap will be smaller, approaching, but not equalling, 100% mating. Stacked o-rings will simply reduce the quality of the mate, resulting in wobble and mush.
So, the bottom line is that o-rings are effective at reducing noise but not at reducing key travel. The most effective solution would be to take apart the switch and insert a piece of hard material between the bottom of the shaft and the bottom shell—if that's even possible.