thinking about it if i was quark i would epoxy heat spreaders to the fets and then drill tap a thread in those plates probably do that first though then use a paste and simply screw the heat speader to the esc case to apply presure for the paste to work.

You could even use a bit of angled alu like smhertzog and bolt that to the side of the case in the same manner with it epoxied to the fets.

This way you can still take the unit apart for maintenance.