+2 on the Kelley Starret recommendation. He's a bit...flamboyant...(he does live in San Francisco after all), but he knows his shit. Very very helpful at learning proper mechanics, improving mobility, and rehabbing/preventing njury.
They make one kind for front squats and another for back squats. It is supposed to distribute the weight of the bar out so it's not cutting in as bad, that way it's easier to concentrate on form and moving bigger weights.