I saw this cute guy on Pinterest:
What a fun way to help practice proper pencil grip!
If you'd like to purchase him you can find them here: National Autism Resources. I suspect, however, that this could turn into a fun craft project with an old sock, some googly eyes, and glue. Having students make their own "Writing Buddy" could build a sense of ownership within students, and it might make them more likely to use the little helper.
If your students need help with handwriting, I do have Handwriting RTI packets available in Print, D'Nealian, and Cursive.