[dad911]

If you can't find the archway bead, you can cut standard bead (one side) every inch or so. Use plastic not metal.

Since your openings are different widths, you may want to use a more gradual arch instead of half circles. Keeping the bottoms(and tops) at the same height and vary the radius usually looks better.

I'll second the 1/4" sheetrock. Or score the back of 1/2" sheetrock every few inches perpendicular to the bend, and skim coat the whole arch. Set the corner beads carefully, and use a wide knife to fill in between with spackle.

Use a paint mixer in an electric drill to mix the spackle and it will go on smoother.