I finally found this thread again. I won't pretend to be the guy with the answer here, but stumbled across a possible solution on Youtube during a search for something totally different.
The solution was to use a self adhesive Velcro pad(available at most auto parts stores) on the area the ejected shell was striking.
The solution was to use a self adhesive Velcro pad(available at most auto parts stores) on the area the ejected shell was striking.