I have this problem as well since I have tiny hands. My wrists are less then 5" around, so everyone can easily grab me. My Sensei was HUGE wrists and I can never really get a hold of him and I have the same problem with most of the guys in the class. Like the others said though, Sensei emphasizes staying connected and that the palm of our hand should stay connected. As long as I try to do that, I don't have too much of a problem. If my hand does slip off, I just try to get it (contact) back.
You can do without fngers on this techniquie as well as most! Think and try like you don't have fingers!