If o-rings work to adjust the travel the easy answer would be to fill part of the inside of a standard keycap with something hard. First I thought to fill it with glue but that would shrink so hard to judge the right depth, and it would be hard to get both exactly the same.
Would a stack of slices of plastic from food packaging would work? Once you've got the thickness right you could glue them together to make one solid piece which should give a suitable click.