[Steve D]

Most common plastics have a dielectric constant in the 3-4 range whereas mica is around 6. This means that even if you were to use a stable high dielectric breakdown plastic such as Polyethylene or Kapton of the same thickness, the capacitance adjustment range would be about half. You could possibly measure the thickness of the mica and try to find a thinner plastic that provides the same capacitance. Capacitance varies linearly with the distance or insulation thickness between plates. Since Kapton and polyethylene have very high dielectric strengths a thinner piece could provide the required capacitance and still withstand the voltage.

Steve D