Keep your heels down, and stay back in your seat, and just imagine your butt being glued to the saddle/horse's back if bareback. Also, try to keep your lower leg stablized since it should naturally be more movable than your upper leg, and your leg as a whole will follow. Picture it just like you still have stirrups, try to keep your weight pressed downwards towards your feet so you'll have more pressure against your legs as well.
Focus on keeping your heels down and inside leg on with an open knee. Try that :)