Just received my Spigen case and the S21 Ultra SPen is still on the way, somewhere... So I tried to fit in the Note SPen which is smaller and it stayed there.
Spigen Liquid Air S-Pen Case - Samsung Galaxy S21 Ultra - YouTube
Best Spigen cases for the Samsung Galaxy S21 Ultra - SamMobile
Spigen Thin Fit Samsung Galaxy S21 Ultra The BEST Minimalist Case! - YouTube