There is many ways to frame a bench seat.. Here is a pretty easy design
Use furniture grade 3/4 plywood for sides attach a 2x4 every 24" oc. on top to support top. Just screw/nail through plywood and toe nail it into cleat against wall. Trim all edges with 1x3's for a finished look.(corners,top,bottom, against wall, 24"oc vertical would look good too) used 3/4 plywood for top too. if you want you can attach it with a piano hinge for storage.