Gary,
I went to my local plastics warehouse and bought a sheet of plexiglass. Cut to fit the rear opening over the motor. Screwed it down, let the motor heat up the plexi until it sagged then quickly open the clip and push the plexi outward to creat a slight curvature. Immediatly close the rear clip and let the motor heat up the plexi in this position for about 10-15 min. Let it cool and the plexi will maintain it's shape thereafter /ubbthreads/images/graemlins/cool.gif
I did experienced the plexi loose it's shape when the engine overheated once and created higher than normal heat, needed to be replaced. With that one exception the plexi has worked out pretty well. I think I spent about $50 in material, enough for two cuts.