I don't know about plastic covers, but if the towel/washcloth is too low class for you (although they do work and are available in many colors) you might contact the folks at www.radiocovers.com
about custom making you one of their wool felt covers.
The washcloth is cheaper.
Lon - W3LK
PS - I use the radiocovers product on my Collins gear, the rest of the stuff gets towels.