Warren in Lancaster, PA
>I think I would make a segmented molding: at least eight arcs or maybe more. If you try to use only two or four pieces there will be much more trouble fitting it to your opening, which may not be perfect. Also long arcs will have more end grain which will be difficult to work with no matter what method you use. Short segments have flatter grain. You could put a small decorative block or "keystone" at every joint of the arcs.

I would mark the arcs with a pattern, saw and then smooth with a spokeshave to the lines. If the wood is hard enough, you could use a scratch stock for the molding (does not work well for pine or poplar). I find it easier to mark the edges of the molding with a marking gauge to delineate the molding and carve with gouges.