Just thought id stick two photos up of the semi-hi roof i cut apart just to show how i did it.
I left a couple of inches of the metal roof all the way around the gutter line. This gave me a bit of strength and something to weld to.

This shows the strengthener parts i put in above the drivers head.

I guess you could just put cross bars above the drivers and passengers heads, in-line with the gutter line going from side to side and then make that a roof/shelf.
Hope this makes it a bit clearer what i did.