For a shawl that will stay put on your shoulders, adapting a Christmas tree skirt pattern would work nicely. Just make sure that the hole at the top of the keyhole is at least 17 inches in circumference.
Another shape that stays put well and could easily be done with pockets is a V shape. I hope I can describe this because I don't have a diagram!
Imagine cutting three 25 inch squares out of fabric. Designate one square as the point square and sew the other two squares to adjacent sides of the point square. And there you have the V shape.
This shape stays in place nicely on it's own or you can easily add a button and loop closure (or button and buttonhole or whatever).
Rectangular stoles are tempting to make because the shape is so easy but many hand knitters have discovered that the shape requires the fabric to bunch up around the neck. This is not a problem for a very thin, soft, drape-y shawl but for a thicker shawl the folds around the neck are often uncomfortably bulky. A V shape or a split solid shape (square, pentagon, hexagon, octagon, etc) with a keyhole neckline lies smoothly around the neck and shoulders.