While being towed aloft behind an airplane, the single-seat glider’s cockpit canopy inadvertently opened. The glider pilot released from tow and attempted to close the canopy to no avail. Around 20 minutes later the pilot entered the airport traffic pattern for landing. While on final approach, the pilot was unable to activate the dive brakes to shorten the landing distance due to her left hand being used to hold the canopy down. The glider crossed the runway threshold around 300 feet above ground level, and continued past the end of the runway for approximately a mile before an off airport landing was performed. During landing the glider’s right wing impacted a fence post, resulting in structural damage to the aircraft. The glider came to rest in an upright position and the pilot was able to exit unassisted. The reason for the inadvertent canopy opening was not determined.