I don't see where you'd need to put spacers in there. I'd just put a layer of backer board over the existing sheetrock and put the SS back on top of that. You could then either get an SS edging or just fill the edge of the backer board with plaster and paint the elevated edge. It looks like your hood already has an inside lip on the back wall, so that would just bring the back SS panel out even or close to the hood edge.