Home Depot and others sell small fiberglass repair kits for automotive and boat repair. For such a small hole it would work very well. I would suggest wrapping the outside of the tube with tightly stretched wax paper as a release agent. Then work from the inside to add the fiberglass resin and either cloth or light weight mat, depending on the tube thickness. Don't try to fill the hole all at one time. Build up the thickness with several applications to give greatest strength as well as control of finished look. Once sanded to blend in and painted you shouldn't be able to see the difference!
Walt