A bloody big bolt , seriously mine is just a 3/4 bolt with nyloc, works fine, when in play the majority of load is on the dummy ball, important to spread the load from that over say a 150mm section of the extension if possible. Load at the rear (hinge) is more on the bracket than the pin. important that the hinge is designed so as to keep the extension standing up straight and not allow it to lay over and twist when under load, my old mans setup used to twist a bit so we welded a couple of blocks into it so when its in use its tight against the trailer frame to stop the twist.
Scott