I would like to hear how people implement mounting and light traps for film holders on homebrew cameras.

I am currently making a 5x7 box camera and am using picture frame moulding I happened to have with the right rabbet depth to fit the 5x7 film holder snuggly. Basicly I made a three-sides picture frame that the film-holder slides into (I made ground glass same size with spacers glued on the glass). I haven't figured out yet what to do about the 4th side regarding light traps. What is the aluminum wedge for on film holders (wooden ones)? I'm guessing with a spring-pressure method it pops into a groove on a standard camera back, perhaps for registration (uniformity of isertion distance actually).