Bow Hold Buddies. things4strings.com
At the NAMM show in Nashville I ran into Ruth Brons. She's invented the ONLY bow hold trainer I have seen that actually works. And for varying size hands! This is for a sort of generic "Classical" grip - which I sort of play with, unless I'm feeling odd. But for beginners or those desiring to get a really nice, relaxed, conventional grip this is absolutely the best I've seen. I'm very impressed.
I'd like to get more feedback on these, if anyone has seen them. I may put them in with my beginner outfits in the future, if I get such a thing organized.