I wanted a simple one-piece sample spool that I could print quickly. So I sketched this up. It was designed with the “complex vase mode” techniques highlighted in this video

There's a version with holes for the filament ends and one without them.

It printed nicely for me with .3 layer height and .6 layer width.

NOTE: I recommend slicing the .stl file yourself, the .gcode file was uploaded just to give an idea of potential print times. I print relatively slow so I'm sure it could be much faster than my example.