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I would say most of the difficulty you’re having keeping the knees back is a function of the narrow squat stance you’d have to use to keep a soccer ball between your knees. The only way you’d be able to keep the shin super vertical would be to hinge way forward at the hips to counterbalance.
With squatting in general, the maxim that Starrett usually throws out is “knees as vertical as possible for as long as possible”. Like Thor said, just make sure the knees aren’t the first thing to load (hamstrings back first), and I would add that the end of your toes is as far forward as I’d let them go (excepting pistols).