Porcelain thermally conductive insulators ?
 

N_Cook Inscribed thus:
1.8mm slabs of porcelain under TO220 devices.

Insulating ceramics are not porcelain (vitrified kaolin clay)
but either alumina (white, very hard ceramic) or beryllia
(usually tinted pink or purple, and toxic if it's made to dust).

In neither case is it advisable to remachine the hole...