A flat black painted fender washer should do the trick. Probably thin enough when you think of it as scaled to the camera. If you really want it thinner than that, get some sheet brass from a hobby shop and drill the appropriate hole in it, and hit it with the flat black can.
Ah, a washer! Such a good idea, thank you! I think you're right in regards to material thickness in reference to the size of the room. Would you be able to suggest any types of thin woods that would work well?