Consider mounting the MOSFETS directly onto a large heat sink, insulating the heat sink from the chassis with discrete, heavy duty insulators, and surrounding the heat sink with a shield to prevent accidental shorting to the chassis by tools and misplaced junk. The shield could be a screen, allowing free convective cooling, or a tube directing forced cooling air over the heat sink.
Direct contact with the heat sink using a heat conductive compound is always thermally superior to the best electrical insulating hardware.
awright