For inspiration, I've used Adney's work, Bark Canoes and Skin Boats of North America which has some fantastic sketches of various styles, like the 3 Passamaquoddy styles in the image. Typically these elongated grips allowed for a more natural grip posture so that less correction and wrist strain is produced while paddling...a definite plus while paddling solo.
Doug Ingram's site, has a great section on various grip styles that he's produced over the years.
One of my favourites is also the also the simplest to make...the elongated Maliseet grip...basically an elongated (up to 12 inches) flattened rectangle that's cambered to fit nicely in your palm when placed along the side. These grips can also be grasped along the top and don't need any rasp work or other carving instruments to shape other than the basic spokeshave. It's basically like carving another smaller, thicker blade. For decoration, I used a round file to shape out 2 semicircles along the bottom edge of the grip.
Maliseet grip in the cut out blank...basically an elongated rectangle
Cambering the grip with a spokshave