I'd drill a couple of holes near the edges of the back panel, across from one another and clamp a couple of bolts in a vice so the ends fit in the holes and use either a strap wrench or a pipe wrench to "unscrew" the outer shell. Or maybe just one hole right at the edge and use a prybar to lever up the back panel if the two holes didn't work.

Edit, you try heating the back panel on the edges with a torch to soften the glue.