My windows are shed/playhouse windows from Amazon. I love them! They are the perfect size and were affordable. I marked the center of the front wall for my rectangular window. This window does not open but will add a lot of light and make the camper feel larger.
The window is 10 inches tall and 23 wide. I cut the opening with the jig saw again. On the inside I framed around the edges with more of my ripped 2x4s.
This helped sturdy up the front wall and gives a nice finished look to the window frame inside. It also adds backing for the window screws to attach to. This front wall will be insulated with some foam board and will have some wiring for 12v reading lights installed later.