I'm planning to build a bench for my in-laws. This is the basic plan: (it's 150x70x40cm). The body will be made from 2.5cm by 12.5cm wood and the legs are 10x10cm.
You want the back to be firmly braced by the bench base. To do so extend the back bench legs up to the height of the top of the back and connect them with a horizontal beam. You can then set the angle by extending supports at the sides and two center supports down from the top to the seat support framing.