is this in a bay window area? Are you in a place where the perpendicular walls are on either side of the window? Is this free standing or hooked on to a long wall under a window?
Need more information so I can suggest on how to build it. If you have a perpendicular wall one either side of the window, you need to install several sturdy cleats or mini ledgers on which to anchor the bench to. But if no perpendicular walls, then we go to plan B.