Is there a way to make a pencil-gripper?

Just wrap something soft around it. I imagine a piece of an ace bandage or moleskin would work well

Blu-tac (or removal adhesive gum of your preference) can do the job in a pinch.

My violin teacher told me about rubber cement that I could put on the sponge rest I had as a shoulder rest for my violin at the time. She said it was rubber and soft, and would make the rest not slide from under my chin. I think if you take rubber cement and do the same thing to a pencil all around it, it would be the same concept as a pencil gripper.

Wrap it in tape.

@ChazMaz is right… tape, particularly masking tape or duct tape is a great quick fix. Wrap it several layers thick so that you have a nice cushion.

Wrap a rubber band around it, or electrical/duct tape with a cotton ball underneath.

I would suggest Mole Skin, wrap it a couple of times…. I’m left handed and the way I hold my pencil always calususes my middle finger up pretty bad, Mole Skin works wonders and it’s a soft plush material so it’s comfortable too! It’s usually used to help prevent blisters on feet, so it’s great for hiking and stuff too. Here are some images from google for you.
http://www.google.com/images?um=1&hl=en&rlz=1I7ADFA_en&tbs=isch%3A1&sa=1&q=dr+scoll%27s+mole+skin&aq=f&aqi=&aql=&oq=&gs_rfai=

Electrical tape.

I’ve done the rubber band thing and it works great. Just wrap one large or several small rubberbands around the spot on the pencil or pen where you need it.