A Harness for a glider would be like tying your wrists to your thighs and expecting you to move around comfortably. It could damage the glider's patagium (gliding membrane) that attaches at their wrists and ankles.
A SAFE way to take your gliders out side is in a zippered Bonding pouch that has screen for ventilation. You could carefully put your hand into the pouch to pet them, but you cannot let them out to roam freely.
Most gliders do not appreciate being outdoors when it is light out any way - that is their time to be sleeping. They prefer to stay in the dark during the day time.