[@knowltonjayne]

Yes, the design is parametric. In the OpenSCAD source file, take a look at lines 11-12:

// ** the two main parameters **
// may produce a useless design, so change at your own risk
ball_diam = 60.0; // 60 mm is default; can go to 45 if wall_thick reduced
pen_diam = 10.5; // should fit most pens; 10.5 mm is default

You can easily drop the diameter down to 57, and `pen_diam` seems to render a good result even if dropped to 5 mm. You *may* need longer screws, though.