For a project I needed a simple gripper to grip a tube and twist it. I printed this simple model (first without the rounded edges, ouch!) and it worked splendidly. I attached the OpenScad file so you can change the inner radius.

To twist the tube I had to add some grippy surface in between the surfaces of the gripper and the tube, otherwise it slipped. I used a simple elastic band, but anything rubbery will do.

Have fun and I hope it is helpful.

p.s. I will post some photos of the printed gripper