Do your toes turn out?? Usually one problem is linked to another. It is very hard to have your thighs/knees out when your toes are pointing forward.
By the way, be glad you have loose thighs rather than someone that tends to grip with the thighs and knees!!! We all aim to have a looser upper leg
You just need to learn to control that looseness. It is not about forcing the issue, but just making minor adjustments such as concentrating on keeping your toes forward.
Taking your stirrups away will encourage you to grip with your upper leg, and to a degree this will be good for you, but you do not want to go the other way and start gripping. Gripping with your knees will block your pelvis, take your lower legs off and swing them forward. This will effectively block your 'go' button. You don't want that!