How can one mount PCB-mounted TO-247 devices to a large plate heatsink?
I did this once: -


Use a strap of sturdy metal to push the transistors down onto the base of the heatsink.

You'll need to drill holes in the sides of the heatsink to hold the strap in place.

Legs of transistors point upwards and solder into PCB.

Transistors and PCB can be removed from the heatsink by undoing screws holding strap.