Could you have some planters up there with plants in to help hide you.
That sounds like a great idea. I would use artificial plants so they are always there any time of the year and no maintenance. I would start with a small amount of large leafy plants to form a small area where you lay out. Then over time get more plants and increase the size of your sunbathing area, until eventually you have the entire roof covered in plants and can walk around all you want totally nude and be covered by plants.
You could put up a false roof along the front that looks like a pointed roof but with no backside to leave plenty of room. I knew one couple that even put their hot tub on their flat roof. In college we had parties in a downtown building that had a flat roof. We stepped out of a friends apartment to the flat roof.