The mixture you are using is liquid at/below the boiling point of water, correct?

Blue-Board insulation is fire-rated and easily carved to fit.
Balsa wood is easily carved, but not fire-rated.
Dried (heated until completely dry) Bar-sand is cheap, but heavy relative to blue-board or balsa. It is also not electrically conductive, fireproof and may also be mixed directly into the potting mixture.

Note that the potting compound is not meant to insulate against heat, but should be able to absorb and dissipate heat to the housing.

Good luck with it.