Homebrew PCBs

What I've done is just make a big hole where the heatsink goes, big
enough to solder the heatsink from the other side after the part's other
pins are soldered.  With a big blob of solder paste in the hole and a
bit of copper wire, you can get the properties you need.

If the remainder of the component-side copper of the heatsink is
pre-reflowed to the chip, that's even better.