I would build a frame out of two by fours, screwing and gluing them together.
I would finish it in hard wood veneer plywood and miter the corners. That will also be your seat/lid.
Stain & polyurethane to your tastes.
Screw the 2x4 frame to the floor & the wall, after you have located some studs.
Hello, here is a video instruction, hope it helps you
I am not exactly sure what you mean by a window bench, but this is one my husband recently built for us. I wanted something to hold plants and to fit exactly. We like to recycle and had this formerly used wood. I wanted it open underneath (for flexible storage) but it could have been enclosed which would have added strength.
Here's how we built one pretty simply: https://www.frugalfamilytimes.com/2017/03/how-to-make-built-in-window-seat-from-2-ikea-nightstands.html
I believe this is a Project you can find at top of hometalk home page - left hand side