You can use the foam gasket material that is used to seal around doors/windows, if you want to go the cheap route. It usually comes in a roll in various widths/thicknesses to suit your needs.
If people would squat to full depth then they would be less likely to hurt their back.
You wouldn't even think about going ATG with weights that you cannot properly handle. People hurt themselves 1/4 squatting with weights higher than their abilities.