1.5mm? So you install them inside the switch, to preload the spring? Wow.
Anyway, I like low-force switches, so I only use o-rings to soften the bottoming out, but you need to find a durometer of 50 or less, otherwise you get the hard thud you described. The o-rings I'm using install very easily, on the keycap stem. For metric the ideal size is 5mm inside diameter, 1.5mm thickness. They have an almost imperceptible reduction in key travel, less than half a millimeter. For more info check the thread pointed to in my sig.