Brian, if you know someone with a router and a pattern cutting bit (flush cut bit with a bearing on it), you can use your old negative holder as a template. Once the plastic, masonite or whatever is cut to the rough size, you can use the router to make a final pass and the bearing will follow your original exactly.

It will take someone with a bit of experience, but this is a fast way to produce a number of parts that are identical, fit perfectly and are exactly like the original.