When we have screws with stripped heads on our planes (Navy planes - happens very often 'cause panels get removed and re-installed a lot), we drill a small spot in the head and then tap an "EZ-out" into the hole. The EZ-out looks similar to a drill bit. When it's tapped into the drilled hole, it takes a bite and then you can use pliers, vise grips, whatever to turn the EZ-out and break screw loose. They come in different sizes and you can probably pick them up at Napa or a hardware store with good tool selection.
-David
