After being aero towed to 2,000 feet agl, the pilot was unable to find adequate thermal activity and returned to the airport and entered the traffic pattern at 800 feet agl. The pilot encountered a high sink rate on final approach and landed short of the runway. The glider collided with the airport boundary fence. The pilot said that the accident could have been prevented if he had shortened the base leg turn. He also said that the glider did not experience any preimpact malfunctions or failures.