When I build a cradle out of pvc, I make a base for it that extends beyond the front and back of the plane, so if it slides around, this base prevents the plane itself from hitting the truck. I've tried facing the planes "into the wind", but it seems to buffet them more, at least in my truck (full size GM). Putting them tail first keeps the aerodynamic surfaces out of the slipstream (except of course when I pass a transfer truck)