I tried to design an ellipse with QCAD Tools - but I stuck .... maybe someone has an easy solution for me

Case: Imagine a model wall build with round dowels - like a Blockhouse,

Now I have to cut the top round dowels under 30° to get a gable shape,

In an isometric this cut has to appear like an ellipse on both sides,

Right side was easy to do with QCAD - maybe not absolutely correct but good enough

Left side .... a disaster I have no idea how I can make it correct looking

A square dowel shape is used as the construction base in the isometric.

I know how to design a correct ellipse step by step - that is not what I'm looking for! I'm looking for a fast and "just good enough looking" solution in QCAD!

Any Ideas?