Best practices for a phenolic spacer design to facilitate heat transfer?
Since I want to shed heat rather than contain it, what are the best
  ways I can to improve the spacer and/or PCB to better facilitate the
  transfer of heat from the PCB to the spacer and case?


Try and use a different material if possible, I'd imagine that you chose phenolic because of its electrically resistive purposes. If heat needs to be conducted away from something copper and aluminum are your best bet. 

Thermal resistance is determined by the material, the surface area and height, so you need to change one of those parameters to change the thermal resistivity, usually changing the material is easiest. Making materials that are not thermally conductive thin is another option.  

If you really do need to move heat out of the PCB and not be electrically connected thermal grease and a heatsink adjacent to the mosfets would be another option.