This is also a bad habit I have, ingrained over 20+ years of riding, opening my chest downhill.
Two other drills that are similar to above have helped me:
1.) A bit more of a dramatic body positioning exercise but something that helped me as I had a LOT of ingrained muscle memory so I almost felt like I needed "bigger" steps. I placed my downhill hand (I ride goofy, so right hand) near my left shoulder. Think of it like placing your hand over your heart when saying "The Pledge of Allegiance" but hand placement is farther over. When riding with a small backpack, I'd go as far as to grip the shoulder strap on my uphill shoulder (left). With my other hand, I would bring it around behind my back with the back of my hand resting against the small of my back - like a one-handed Parade Rest. Riding like this helped to keep me from opening my chest downhill. The downside for me was my right arm across my body (Pledge of Allegiance) tended to make it harder for me to turn my head fully 90 degrees to look downhill.
2.) Like the one above, instead of placing my hand on my thigh I lightly grip a section of my pants, between the cargo pocket and top of thigh.