As Beth correctly points out, the purpose of the saw dust was not so
much to RAISE the cab as to CUSHION it, but, of course, through
time, moisture will take it's toll. But it takes a surprisingly LONG
time for it to become loose and sloppy in the setting.
In my store, we made many, many pieces with cabs or cab-type
lapidary materials, some of them more fragile than we would desire
(the stone being fragile, that is). Even some of the standards like
lapis and sugilite, not to mention opal, tend to sometimes crack
when worn because they have inherent and invisible seams of weakness
in them (or they just get whacked too hard!).
What I did was use one of two readily available products, both
available at the auto supply store. One is a heat resistant gasket
material available in sheets of varying thicknesses, used, of
course, to cut out gaskets of the desired shape, generally for
intake or exhaust manifolds where temps are very high for long
periods. Permatex has made this material for many years, gearheads
know all about it. It will resist heat of a few hundred degrees, is
available in different thicknesses, is an excellent cushioning
material and can be cut with scissors. It accepts many adhesives, as
Permatex also makes a liquid gasket/sealer that is flexible and also
provides cushioning at the same time. It is quite viscous and can be
applied in any thickness desired. If you rough up the plate that the
cab usually sits on, this stuff will adhere VERY strongly, and
sticks to the cab as well. We usually applied it to create about a
cushion about 1/8th inch thick and lightly pressed the cab on top.
Bezels/prongs were turned after eight hours when the material was
completely set. We never had a failure of the material and never
suffered a cracked piece.