What Bigbat said. EVA or insulation foam would give you the best size to weight ratio while still maintaining the rigid form. As for the harness, I recommend some flat nylon webbing, at least an inch wide to give better weight distribution for prolonged periods of wear.
You could also try sintra or Worbla, but given the size of these, those would be far more expensive options. If I were doing it, it would be in foam.