These surplus high power PAs using 4 x MRF151 were originally used in watercooled industrial VHF drivers. The devices and some of the other components protrude about 2mm below the PCB lower surface.
To minimise the amount of copper required, I decided to make a heatspreader to go under the devices and make aluminium spacers for the rest. I also decided to make a fixture to hold the spreaders so I could cut neat pockets for the devices. Not sure that was a sensible approach. It certainly meant I had to do some tricky machining.
I have some more to make, I won’t be doing the fancy pockets this time, just transverse slots.