I have the Model D (and the sport Model D as well). I find them difficult to pack with because the material is quite hard, and it tends to turn sideways when I move. I have to always reposition it...
the best STP I've packed without a harness is the EZP. It stays very nicely in place (I wear boxer briefs).
I've been packing with it during Martial Arts training, with all the movements and close-contact combats, it sits nicely in place.
I've also packed a Peecock but with a self-made harness (just an elastic band with a ring type), and have been fine with it with all the movements. I don't feel the harness at all, but just sometimes a bit worried others might see the harness...