There are several different designs existing for z-axis Prusa Mini spool holders. Main types are vertical (spool is hanging or rolling in standing position) and horizontal (spool is lying).
The main disadvantage of side-mounted vertical spool holder is weight distribution, the center of mass will be far away from z-axis (spool is on the side of the z-axis), making the printer wobble. The main disadvantage of top-mounted vertical spool holder is that it requires a lot of vertical space for different spool sizes, and can't be very compact.
The main disadvantages of horizontal spool holder are friction and instability of filament supply: it is harder for spools to rotate because of friction, and you need support of ball bearings, and you also need some sort of filament feeder, so that filament doesn't unroll and drop down by itself.
Diagonal holder is designed to address the problems above mentioned. It is like a subtype of vertical spool holder, where the weight is redistributed to move the center of mass above z-axis, and the printer will be stable. See the photo, how it works with spool of 1 kg Prusament, and it can stand on the desk without falling.
At the same time, it is very compact and printed as one piece, without need of additional mechanical parts.
This z-axis foot is recommended for supporting z-axis vertically: https://www.printables.com/model/360734-prusa-mini-z-axis-foot
The author marked this model as their own original creation.