Christopher Tidy writes:

but I just thought I'd ask here in case anyone can
think of a source for a small piece of semi-silvered glass or plastic.

Inexpensive if you can find what you need:

http://www.surplusshed.com
http://www.anchoroptics.com/

Has what you need and charges accordingly:

http://www.edmundoptics.com/
http://www.mellesgriot.com/
http://www.thorlabs.com/
http://www.newport.com/