I am going to TRY and not get too technical here. My chance of success may be less than ten percent but I'm still going to try.
MOSFET transistors have a metal tab and backing plate on them. With a hole for mounting. But that plate is connected internally in the MOSFET. So whatever the MOSFET is mounted to may need to be electrically insulated. I refuse to go deeper into that part.
The issue is this: The MOSFET may or may not be INSULATED from that aluminum plate heatsink it is screwed to. With a thin sheet of material called Sil-Pad. No way in hell is Sil-Pad as good a conductor of heat as clamping the MOSFET directly to the heat sink with zinc oxide. I have no idea of what reality is from these images.
NEXT: The heat sink may or may not be insulated from the gizmo's case with sil pad. This is sort of doubt because of the cost of Sil Pad material.
Here's what I'm driving at:
If the MOSFET / heatsink / gizmo case SANDWICH isn't that bad a thermal conductor (by some miracle) then pasting a larger heat sink on the outside of the case might be worth looking at. Lots of if's here. It is a long row to hoe between the MOSFET internal junction and the proposed external heat sink.
I do not know the answer to this. It has to be tried and an IR pyrometer used to establish Delta T (differential temperature) between the 3 prongs on the MOSFET and what the heatsink temperature is at full song. Maximum output.
I can only hope that all this did not cause eyes to glaze over in bafflement...
