If the marked “walls” are vertical and the and the marked corners connected, you can extrude a column based on parallelogram over the heigth of the another bodies.
After as Tiger Mike wrote you can make a construction plane with the three point. Project the triangels line to the Cp , then draw a bigger rectangle and extude.
Subtract this body from the previus body.
I hope it is help.
Regards,
A
