Is very simple super and it worked for me really well, I only did it once and it was one of those begginners luck results. Once I saw on a TV program back when they had TV programs about photography how one professional photographer made a composite by putting a glass in front of an 8x10 and then placing masks he would cut. Sort of doing what Donald is doing in the darkroom or the photoshop thing. But since I did not have all that money etc, I thought, well why cant I just place a clear filter and fill the area with a marker. One of those big tip indelible markers. What would you know that it worked great! I had this scene where I was shooting a difference of 13 stops, and I wanted the walls inside as well as the stuff outside the "hole" in full sun to have detail. So I tried it and it worked, and this was on 35 mm.....So if you can find anything that would block light competely you can draw it on a filter and make two exposures. one for your foregorund and one for the sky...Actually what I would get now would be a can of spray paint and then remove the paint on the part you want to come through. Of course you would have to carry the paint ,solvent and q tips, maybe is not a good idea. How about one of those black grease markers?