The Pencil Grip Ergonomic Writing Aid, Assorted Colors, Polybag 6 Count (TPG-11106)
by The Pencil Grip
is the one Tristan recommended on an older thread and I purchased it for my ds9 who keeps wrapping his thumb around the front and this helped him so far. He has used it for 2 weeks now. Occasionally I have to remind him to keep his thumb in place. If he was younger, I maybe would have needed the crossover grip. But since he is older, he is more aware of his finger placement.