John,
Just as you, I am involved with this matter now. The information that I have been able to gather from Jorge and others is that what works is a cloth of sufficient size to wrap the perimeter of the camera back (secured by elastic with the two corners attached to each other). This makes an enclosure around the camera back. Then the other end would have the two corners not attached to each other and a distance of approximately 6 feet from the point at which it attaches to the back of the camera to the opposite end. I figured that mine would need to be about 90" by 72". (the perimeter of my back is 85"). I figure that this will allow it to double as a tent. LOL