VVR - Heatsink mounting and size?

[CrunchBerries]

1. What thermal pad do you suggest to mount a mosfet (TO-247)? I do not want voltage on the heatsink, just heat transfer.

2. I got a 3C/W heatsink for a IRFP450. The input voltage will be about 430Vdc from a Hammond 272-FX, and I will have it drop to 330Vdc with the mosfet. The load will be cathode biased EL84 a la AC15 - so about 114mA full tilt is what I gather from datasheets. So I figure this will result in a 11.4 W dissipation, and since my heatsink is 3 C/W, then that would mean ~ 36C of heat, plus ambient - say 40C for a hot summer day - brings me to 76C. I am guessing this is ok. But this is also for an open air scenario. I want to place the heatsink inside the chassis for safety reasons - but I am not sure if this is still a reasonable heatsink in a closed chassis. I was thinking of using a metal grill, near the sink, sized so the sink can easily fit through the hole required on the chassis - I just wonder if this will suffice?

Thanks