[CAD_CAM_EDM_DRO] Hotwire CNC on mill

I have a long, thin block of polyethylene foam which needs both internal
and external cuts parallel to the long axis. Basically looking for a
cylinder with a hole in the middle like a pipe insulator, and there will
be a slit where the wire entered to cut the internal hole. The slit is
undesirable but necessary. The long axis will be put on the X axis but
it is bigger than the table's X axis, maybe 3 ft even.

My approach for this one is to replace the Taig cutting head with a
hotwire bow spanning way out on the X axis. Stiffness is going to be an
issue but there's very little resistance force from the work in
speed-controlled slow hotwire cutting.

So the Taig can enter a block of foam through motion on the Y axis and
make the cut profile with a combination of Y and Z moves. The X axis
is unused. This seems workable. If it proves to be a good idea maybe
I'll build a more "proper" rig.

Anyhow, my question is this- how would you CAD this shape out and tell
Mach3 (or some other software approach) to make it? This is a different
approach than we see for the traditional milling. The toolpath is kind
of nonsense for standard 3D milling in that the top half of the circular
cut is directly on top of the bottom half as well as the internal cut.

Danny