What about keystones in the center? It think it would be cheaper to buy stock length, cut the center, then cover the cut with the keystone. I wasn't sure if keystones would look right on a hardiplank house. I always imagined them for brick houses in place of a masonry keystone. Any thoughts?

The only kind of trim that should ever be placed below a window is a sill nosing that projects beyond the face of a nail-fin window's frame/sill and extends horizontally under the jamb trim kind of like a real wood window.

ATW makes all kinds of PVC window trim.