i've probably mentioned this before, but when i was a teenager, i made my own HV caps out of plate glass and copper foil. i had to do the capacitance calculations, but many modern DMMs have a capacitance function on them. the dielectric constant of glass is between 5 and 10, and the dielectric strength is about 115kv/cm. teflon has a higher dielectric strength (450kv/cm), but it's dielectric constant is about 2.1. many of the old "wireless" texts have details on proper HV capacitor construction. if you like "old school", you could even try making Leyden jars.