I would second the jig and an aluminium core PCB, as this can easily and cheaply be done to precisely fit the device pinout and even have slots and other things, like mounting tabs, milled in for essentially free during production, plus you have quite good thermal resistance from the copper to the base, and can even integrate other parts in there simply, like a heatsink thermal sensor, power devices like resistors that need heatsinks, or even just test points. Then you only need to have a few holes to mount to the main heatsink with a thermal pad to lessen the tolerances needed ( thin pad is a lot cheaper than tight control of surface finish on 2 parts) and spring clips to hold each device down to the heatsink. That way rework is possible, and sevicing or repair is a lot easier as you can remove the individual parts easier. just use a thick copper on the aluminium core PCB, and make the pads larger with a solder mask to keep the solder under the TO packages, and in later versions you can use a SMT version ( lower cost immediately there) and simply have some surface mounting pins, to keep the footprint for the leads to the main board the same, to save a respin.