I experimented with some elastic things in my apartment but they didn't offer enough resistance to do the exercise properly.
I hooked the ends of a 36" bungee cord and looped it around my bookshelf. Then I slid my left thigh through.
I stood with my left leg straight and my right leg slightly bent with my right foot behind me. I slowly started doing one-legged squats with only my left leg.
The first few iterations, my knee was going in and I could not see my left foot.
Instead of moving further inwards, my knee did eventually started to resist and after a few squats, it was noticeably straighter.
It was almost like magic. It's amazing what a little resistance can do for a Green Girl.
Dr. Gonzales recommended I do this exercise once a day to help reprogram my leg to bend properly.