I bought my fabric from Wal Mart as well. Mine is similar to David's in that the end that fits around the camera is tapered some. About 6 feet long. black layer on the inside and a tan layer outside. The tapered end is stitched together for about 18 inches to block light from coming in the bottom.
As for attaching it to the camera I stitched an elastic band in the camera end. It did not turn out as tight as I wanted it to, giving me encouragement to someday add a 5x7 as the dark cloth would fit. Total cost was under $10.