Thanks for all your tips guys. I am originally from Canada and Radio
Shack did often have all the things I needed that were a bit off the
beaten track. The gizmo. And yes, it is the MIC cover I need, not
the earbud cover which I can find in plentitude online..

I have made a Tiny Url as a link to an approximate model. The opening
diameter of the foam cover is no more than 10 mm.
http://tinyurl.com/2trvs4