I fixed this problem was some great high school kids by having them stand up when we tuned. As they got taller and older they still leaned the bell on their leg, which also meant bad posture and band hands (I saw you already addressed the hand). I showed them the tuner when they tuned, they saw the difference and they immediately changed the way they held the horn.

Not sure if it’s the solution you need, but it’s one that worked for me.