My 3L flasks kept sliding off of common cheaper stir plates, so I designed a press-fit support that ‘cradles’ the flask.

Prints without supports.  I've tested printing this with ABS and it's a comfortable yet still snug fit over the stir plate.  The inner diameter of the part (130.8mm) fits what seems to be common molded cheaper stir plates, I have two different brands and fits both perfectly.