I have a project that may need to use dental acrylics to pot a mini PCB
within a plastic housing to form a solid object the size of a stick of chewing
gum.

Any potting compound that you use will shrink when it cures, worse if it is
heat cured. The least shrinkage will be a filled epoxy, room temperature cured.
There are probably dozens of decent commercial compounds available; do a Google
search to start. If your device doesn't have to be monolithic, consider
elastomeric compounds, silicones or polyurethanes. Even quicker, cheaper and
shock resistant are closed cell urethane foams. HTH

Joe