Got the back door and wall in this weekend.  Door is 36″ wide and made by a local RV door manufacture.  Going that wide let me get away with one sheet of aluminum instead of two.  
Used 2″ aluminum channel and .050″ aluminum sheet with 1/4″ rivets.  Rigid foam insulation.

 

Note to self, don’t use 2″ channel for your back wall as it leaves you no room for the plywood.  The doors are made for 2″ walls so use 1 and 1/2″ channel with 1/2″ ply and it will be flush.
 
The corner brackets on top look like you can take them off and install your sheet underneath.  Well you can’t because they have nuts behind them that are boxed in with channel.  Cut the corners to match the radius with an electric shear and siliconed the seam.
 
The steel brackets that hold the original door spring had to be removed so the interior walls fit evenly.  They are welded and would not come off without getting out the 4″ grinder.