I'm in the process of making a new solid state power amp that uses the old but good "Lateral" mosfet by Hitachi. Since they are in To3 package, mounting them onto the heatsink will need some work. I have decided to make a "carrier block". The output transistors will all sitting on the carrier block and then the block will be mounted on the heatsink.
Machining of the carrier block by a small CNC:
This is a test version which will hold two TO3 transistor. The milled block look like this:
We will need a flat surface to ensure good thermal contact with the heatsink, some touch up is then required.
Not too bad I think. :-)