Well the way I make a surface a plane is to use the rotated rectangle, pick three points that should lay on the same plane. This makes a rectangle that is a least a flat plane to three of the points. Then I offset the rectangle until it covers the area I wish to cover, then intersect with model, and pesto, a flat plane. Note, if the flat plane does not connect to all the edges, you will need to move the plane to connect to all the other surfaces, or modify the surfaces that don't connect to the plane.
Ken