I have usually been able to pull cardboard covers off after softening the tarry adhesive with a heat gun. Some of that tar is really tenacious, but with patience the heat gun usually works. In a couple of cases I got impatient and slit the cover up one side with a razor to speed the process. After restuffing, I glued the cover back together and put the slit side where it wasn't visible.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html