I use 1/4" backs with 4 " to 6' wide 3/4" thick stretchers top and bottom that pocket screw into the sides of the cabinets from the back. Dado the inside face of the 1/4" back 1" from theh back of the cabinet, put the box together, slide in the back, and attached the stretchers to the sides of the cabinet. Then i square it up and hot glue the back of the 1/4" panel to the sides of the cabinet and the stretchers in the 3/4" space between the back of the back panel and the rear edges of the box. That's standard construction but I add the hot glue bead. If it all cut accurately it self squares but I always check diagonals. For hanging rails you can buy really nice ones and just make the end panel of the outside cabinet of a run of cabinets deeper to scribe to wall and allow for the 1\4" that the hanging rails are.