A fill (or slab, or mesh) with one or more sides curved to the shape you want can be used to subtract from your mesh (from the right angle context menu 'minus' icon that is) with the magic wand.
[It makes sense to make copies of your unmodified objects and your 'cutting template' poly shape in your workshop story or deleted layer or somewhere, because sometimes meshes don't like pointy things or arcs that get tangential to an edge and they will just not generate. And sometimes you only discover that after a while, or after some other change.]
