Bingo, Poole was right.

All you need to do is put a spline drive socket into the inside of the stripped hole.
I think either a #10 or #12 spline socket.

Just put it at the end of a wobble type extension and pound the end of the extension with a hammer and you should have no problem turning the bolt out.

For added non slip assurance, I always put a little valve grinding compound on the spline socket so it bites in even better.

Sounds like Poole knows his stuff.