Self-adhesive Velcro strips (get the heavy-duty extra-sticky kind). Place them strategically in three or four-inch lengths to allow you to stick edges and sides to the dark cloth together. Carry some extra in your bag for emergencies.
Or, you can sew them (or have them sewn) on to your dark cloth. I made my own out of white Gore-Tex and a lightweight black lightproof cloth lining. I had Velcro strips sewn the length of all four sides (correctly oriented male/female of course). I fold it male-to-male so the Velcro doesn't stick to itself when stored, but use it so that I can make a tightly-sealed tube if needed. I have a strip of Velcro on the camera as well to hold the cloth there. The waterproof Gore-Tex is a Godsend when working around unpredictable surf, etc.
Best,
Doremus Scudder
www.DoremusScudder
Or, you can sew them (or have them sewn) on to your dark cloth. I made my own out of white Gore-Tex and a lightweight black lightproof cloth lining. I had Velcro strips sewn the length of all four sides (correctly oriented male/female of course). I fold it male-to-male so the Velcro doesn't stick to itself when stored, but use it so that I can make a tightly-sealed tube if needed. I have a strip of Velcro on the camera as well to hold the cloth there. The waterproof Gore-Tex is a Godsend when working around unpredictable surf, etc.
Best,
Doremus Scudder
www.DoremusScudder