I'm a big fan of Cokin filters. Mostly because I was given so many and so many of their promotional booklets. They have all kinds of filters for doing multiple exposures. The one I used here is the Double Mask A 342. Two filters One with a black dot on clear resin. The other a All black filter is a little hole. In the booklets they recommend (in the 80's) you take a projector and photo graph slides you already had in order to create special effects. I just used a 32" screen.