First, I intend to make the himation longer than the shirt, so that it is at least floor length. (The shirt was ankle length before I hemmed it, but I gave it a generous hem to make certain that it would not show beneath the himation.) I also intend to cut the himation a bit wider than the shirt, so that it doesn't hug my body too tightly when worn over the shirt. As I mentioned, I had to make the gores in the shirt a bit narrower than I intended due to a shortage of fabric; I want to avoid doing that here, since I want the skirt of the himation to have more sweep to it than the shirt does.
In addition, the himation will have a plain round neck, with a slit (probably a very short one, judging from the size of the slit on the shirt) down the left shoulder. To finish the neckline, I plan to retain the square yoke piece, instead of simply cutting a facing for the slit and sewing bias tape or binding around the neck. Because there will be no collar on the himation, I can simply sew the yoke piece to the inside of the body piece, cut the neckhole in the body piece using the hole I've cut in the yoke piece as a guide, turn the edges inward on both yoke and body piece, and whip stitch around the opening. Since the collar of the shirt will protect my neck, any roughness due to the whipstitching shouldn't be a problem.
My only real concern is the trim. It is purest polyester (though with a great design!) and rather stiff in texture; I'm not sure I'll be able to get it to conform to a round neckline. Experimentation will be in order.