We had 20mm (two layers of 10mm) on top of 5mm mild steel on Coyote. Carbide managed to get through it all in a single hit, deforming the armour but not pulling it off the robot. Harder materials can be better but HDPE is still good for some shock absorption. Just do what we did and carry some spare material as you'll probably need to replace it frequently if you plan on fighting spinners.