I don't know if this translates to the body and weight distribution of yours, but for the shorty the only position that really works (for me of course) is having the thing propped up by the bottom of the body on the thigh opposite of my fretting hand, and keeping a straight back, and keeping the thing leaning forward so that at it is engaging the strap. I think it's similar to the classical guitar posture, but relying on the strap and the forward lean to keep it in place since there's not enough guitar body to keep it perched otherwise.