Heat can permeate the exterior as well and IMO Ittou Kasu is more powerful than El Thor. Also I don't know if mantra could predict KS. Mantra only predicts attacks and KS essentially isn't an attack. Aizen also has numerous barriers and has blocked attacks from Komamura, Love and Soifon with just reiatsu. To block El Thor with reiatsu may not be a possibility as it was a stronger attack than those but offset it to a fair extent sure. Combine that with his barriers and durability and I don't think anything but Raigo would have any notable affect and that needs time to perform.