The clothesline should do fine. I just used light-weight, pure aluminum wire. I can easily twist it with my hands around screws.
Are you putting the solar blanket material inside or outside? Mine is all outside (and now two layers thick on the two side-walls and top. I cut it out for the twin sliding doors and anchored that with the screws that hold the polycarbonate panels in place. Because of the close-tolerance of the sliding doors, I have that material on the inside of the doors. The flap is outside though and can be rolled up when not needed. I used Velcro to hold that rolled-up blanket material near the roof.
