The easiest way to create this kind of irregular shape would be with Art of Illusion:
- click on box tool,
- draw a box of any size,
- select Object > Convert to Triangle Mesh menu item,
- select Object > Edit Object menu item,
- In the object editor, move the vertices like you want,
- once done, select Export > Wavefront (.obj) menu item,
- import in Sweet Home 3D the generated file.