Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4934132.
The newer Samsung Galaxy S21 series no longer has an option that ships with an S-Pen. I loved my Note and missed the pen. However, the S21 is compatible with all previous Samsung Pens. Only problem? No place to put it. Sure you can buy a case that has built-in pouches for the pen but the cases themselves only provide sub-par drop protection.
My solution, design a holder for the S-Pen that can be attached to the side of my existing phone case. You can mount this anywhere you like. I didn't have room on the back of my case and since the new phone design places all the buttons on one side leaving the whole left side of the case open for accessories I chose to place it there.
I chose to use the Galaxy Note Stylus Touch S Pen (N950) due to its slim design to minimize the amount of space added to the case design. If you have a different pen I cannot promise it will fit. I have no idea of those pen dimensions.
I recommend printing this with a flexible material to allow the pen to slide in and out easily while keeping a good grip. There is a small lip at the end that helps to keep the pen in place and not accidentally slide out.