The time scale of the failure is directly related to the pressure loading per
unit area and the temperature. There is a modest static load resistance that
must be overcome, then rapid flow. While I don't have the flow constants at
my fingertips, the example I cited of 1/2 inch rod stock (4 inch diameter)
with 350 pounds bias pressure, the rod stock was buried in less than 3 days.

This is not to say this shim method will not work. In my recent polemic, I
indicated that Plexiglas was just fine. Aluminum sheet will work. This
problem is a unique feature of Teflon. If Teflon is being used as a pseudonym
for plastic, then almost any other household plastic will do just fine. (Not
foam structures of course!).

Frank Grunthaner