Remix of Finger Pen Holder (Vertical) in OpenSCAD so it's more easily customized.
The two parameters are the circumference of the finger you want to fit and the circumference of the pen/pencil/paintbrush/utensil you want to use. A fairly easy way to measure either is to (gently) wrap tape-style dental floss around the digit or pen, then mark the overlap with a permanent marker. Remove the dental tape and measure the distance between the marks in millimetres with a ruler: that's the circumference!
If you want a more snug fit, subtract 1 mm from your reading. Dental tape is thin, and doesn't add much to the measurement. If you used twine, you might have to subtract 5 mm from the measurements. If you used sewing thread, you'll find that permanent marker spreads all along the fibres and makes a mark you can't measure with any accuracy at all …
Alternatively, you could make the measurement with Volksswitch's Measuring Stars …
You must print with supports (everywhere, rather than touching bedplate, if you have the option) as the finger grip has a cutout that won't print correctly without supports.
If you're at all concerned about the strength of this design, please use MMC's print settings from their original.
Carefully remove the support from the middle of the finger grip. Remove any rough edges by lightly buffing them with an emery board.
Model construction: intersecting 2D extrusions
I simplified Makers Making Change's design by modelling it as 2D XY and XZ planes extruded and intersected.
The mathematics in the smooth fillet is explained in the Geogebra workbook Sine Rule Fillet.