I designed this S-Pen holder for my Samsung S21 Ultra because the protective case I am using does not feature an S-Pen holder. 

The side walls feature slightly inward-bent edges at the top (see cutaway view of the 220109... version), which keep the S-Pen in place.

I printed the holder in PLA at 0.3 mm layer height in less than 10 minutes and glued it to the button-free left side of my protective case by means of strong double-sided tape.

As always I include the OpenSCAD source for you to improve.

See my other designs at
https://www.thingiverse.com/thinger13/designs