Once upon a time, I saw a picture in this forum of keycaps that some Chinese guy (?) had filled with Sculpey (or other polymer clay) in the empty space above the key stem.
However, that would only work with PBT keys. ABS starts warping at a temperature lower than what is needed for curing Sculpey. Sculpey is also not adhesive when cured - it would be held with friction only.
Btw, Cherry profile keys on the home row -- the lowest keys -- have very little extra space, even the thin-walled PBT keys.