Why would you want to slope the bottom of the wall if you are going to export for furniture anyway? Just slope the top, export, import, and turn it up-side-down!

If you want a tetrahedron you could create that with the Generate 3D Shape tool (tools menu). And there is the geometric forms library from OK Hoff: https://sourceforge.net/p/sweethome3d/d-models/299/

Good luck!