The suggestion has been made to use sawdust to build up the
cabochons. We just recently learned that sawdust (the fine dust)
is carcinogenic. My husband is a woodworker. Because of that, I
have shied away from it. Thought others might like to know to be
careful with it too.
In addition, I might point out that sawdust, leather, paper, --just
about anything organic -- will absorb water and swell. Which could
either fracture the stone or push it up in the bezel, leaving it
loose in the bezel (and apt to fall out) after it dries out a bit.
(And if you think those rings and sometimes also bracelets don't find
themselves immersed in the dishwater or shower, then you are a bit
out of touch with reality!)