Since this will only be in place when you take the shot, it doesn't necessarily have to be the same material as a regular dark slide. Take a look at your local hobby shops and art supply stores for plastic that's the same thickness as your dark slide...it shouldn't be hard to find something. Make sure that whatever you find blocks light, and it should be a simple matter of cutting something to the right size and attaching a handle to it.

I have no idea what kind of plastic modern dark slides are made out of...all of my 8x10 holders are older and are metal. If you can cut metal (for example, with a jigsaw and the appropriate blade) you could make this out of a sheet of metal that's the appropriate thickness.

Good luck.