Hi Involute, looks like the thing that makes your case here a bit more complex is your cut isn't in a straight direction.

I'd probably try building a network surface to use as a cutting object with boolean difference. But on this one side here I don't think it will work to put in a long straight line, because this area of the existing object looks like it's a slightly curved edge and not just a straight line:



So probably you'd need to draw in a line from the top of that red area above and incorporate the actual curved edge as the rest of the curve, rather than a long straight line for that side.

And also the kind of same thing applies to the short segment on the bottom - you'll need to use the existing edge there as part of your cutting surface construction, it's slightly curved. If you just draw in a line there it won't match up 100% .

- Michael