For isolation I would use Aavid insulators, mostly ceramics if there is enough clearance, Aavid PPS shoulder washers and stainless steel mounting hardware tightened to 0.8Nm (M3 or #4-40 allen head screws).
The Aavid insulators and shoulder washers are available in different sizes, depending on transistor packaging and size.
I am attaching an ONsemi paper as well including mounting considerations for power transistors.
For grounding follow this scheme, which shows pretty much the most complex case of a two box design, amp with a separate PSU chassis. Pic shows single ended amps, balanced would be slightly different as the signal do not touch ground, the amp chassis would change so that the inverted signal (cold, pin 3) would go to a second amplifier board instead to ground. Overall idea should be pretty clear however.
Mounting Considerations - AN1040-D.PDF