When I build cabinets, I find it easier to rout the rabbeted recess in the carcass for the back panel after the cabinet is assembled. It’s hard to do this while balancing the router on the edge of the plywood. My solution is to mount the router to a piece of long, flat scrap to act as a stabilizer. I drill a hole through the scrap to provide clearance for the bit, then bolt the router right to the scrap. For a large cabinet, I make the scrap about 4' long and as wide as my router’s base. Make sure the scrap is longer than the cabinet’s diagonal dimension. The stabilizer spans the opening and provides sure footing when I make the rabbet cut.