Trapezium = a muscle in the upper back and shoulder.
Trapezoid = a geometric shape.
Anyway, the sliding portion can really only be a little more than 1/2 the length of the window. The bottom will be flat, and parallel to the top of the door. That will be what the sliding portion actually slides on.
The top and front portion can match the top and front shape of window frame, and that's where the window would seal to.
The back 1-2" of the sliding portion would be parallel to the bottom. And the back portion of the sliding frame would also be parallel to the bottom frame.
Once the window is slid open that first 1/2" or so, it's sliding on the entire bottom edge, but only the top 1-2" or so. The top only keeps the window from falling out.