The final design was a sandwich with metal backed PCBs to route the heat away. The heat from bottom PCB conducted straight to the mounting face and that from the top conducted down the large mounting pillar (not shown in the drawing for clarity). Extensive calculations were done (and documented) to show that the temperature rise was acceptable (in fact there was over 50C in hand on the power device).
The last design hurdle was the fact that one of the 4 regulators required temperature compensation. This was a curve which ruled out ic sensors as they are all linear. Instead a twin precision thermistor was used as the sensing element with resistors to fit the response to the required curve, plus again the bare minimum of components to perform the compensation function.