Isolation for TO-247 Leads at High Voltage (1000V)
1kV is easy, try 30kV but the same techniques are scalable.
Insulated thermal conductors are available but these tend to be trade secrets.

Your options for good thermal conduction and electrical insulation of the drain with low capacitance are limited.  3M thermal tape, thin Ceramic wafer, Mica

http://www.magazines007.com/pdf/High-Voltage-PCDesign.pdf