AvatarNathan Richer

When you descend into squat, the glutes are mostly trying to lengthen so flexing them isn’t going to work. however, it’s the glute medius that helps drive the knee outward and upper leg into external rotation. this is the glute you want to activate which also helps keep your navicular bone off the ground.

when you descend into squat, are your upper legs driving outward into external rotation?
when you ascend out of squat, do your best to squeeze the glutes on the way up instead of relying solely on your quads. this will help too.
when you first train behavior, it will require conscious effort to do it. so arching your foot by conscious means is ok in the beginning. over time, you imprint the behavior and it becomes natural.  try also while standing, flex your glute to turn your knee slightly outward, which should lift your nav bone up too.  practice this whenever standing still – brace in the core 20% but also 20% in the glutes and use the glutes to slightly move the kneecaps outward and feel the torque driving into the ground. 
if your left glute is lacking, like mine was!, try hip openers with bands, as well as smashing the left TFL and adductors. there could be some tightness in those areas which inhibit glute activation.