Hi Guy, the boolean will cut based on the orientation of the 2D curve, so if you need the cut to be angled you just need to rotate the curve to the angle you require and then use the same steps I described above, you don't need to do any projection this time either.

To do the rotation, go to a side view and then you can do it like this:



There I just drag the ellipse over so that it's centered on an edge of your object and then grab the rotation grip and drag it over and snap on to the same edge.

After that do the same boolean steps that I described above - select the solid and run Construct > Boolean > Diff, select the now rotated curve as the cutting object and it will make this result (also see attached 3DM file):



Hope this helps!

- Michael