Jeff,
The only way I know to cut a trapezoid in a wall is to use a series of rectangles rotated to cut the pattern.
You may have to use other rectangles to add back geometry.
I would probably use display cubes in the part editor to create the window.
If you can attach your photo it will help.
If your photo is over the size limit, take it into IrfanView that is in eCabinets and re-size it and reduce the resolution.