Just normal casting resin you get at Michael's. 4oz mixed for the main compartment and 2oz mixed for the gearbox compartment. Fill just to the power switch mounting post height, and since it is a 2.4 Ghz with volume control and smoker on/off on the radio, there is no other opening on the lower hull. Also, the Chally has a separate mount for the actual suspension arm and is outside of the lower, there is no worries of jamming or fouling the suspension arms/springs.
I have to remove the smoker and replace it with a smaller TARR unit as once you strengthen the hull, it doesn't flex anymore and you can't jam the upper/smoker into the lower. Apparently the Heng Long smoker only fits between the gearbox if the lower hull flexes. Go figure.