The PPP are molded from the torso up only. This can cause issues when the plane you are putting them in only has a gaping hole for a cockpit. This was the case with my Shapeways Fokker DR.I:
To solve this problem, I carefully filled the cockpit area with white glue (PVA or Elmer's) and let it dry. The PPP come to a little point at their waist. This works out quite well for my method. Once the glue dries, take a sharp razor blade or small drill bit and drill a small hole into the now rubbery glue. Add a drop of superglue to this new hole and insert your pilot:
That's all it takes to fit these little guys in a cockpit near you!