The point for the ones shown is, that they couple to an aluminium plate to which the PCB is mounted.
Ah yes, that makes more sense.
It looks like a challenge in mechanical tolerancing. Turning it around, what about blocks mounted on the Aluminium plate which make contact with either the exposed PCB copper area, or the D-PAKs themselves. Either way, you probably need thermal pads, as strawberry says, to take up mechanical variations and ensure decent contact [Edit: and electrical insulation].
Thats how its done on the picture I posted. There is a pad that covers both the metal block and the DPAK.
Still, I do believe that this would provide a much better thermal conduction than just cooling the DPAK with such a pad.
What sort of volume? Nearest off the shelf part would be the SMD screw terminals. Wurth do those on tape for pick and place, say 7466303R
Probably not enough to justify a custom part.
I wouldn't even know how to pull that off - even if my metal guy could make this part, the main challenge would probably be putting it in a reel to make it suitable for production.