This is a reproduction and modification of the original agitator for the Paterson System 4 tanks, primarily intended for the first generation System 4 tanks. 

The 110mm agitator stick is ~20mm longer than the original, making it stick out over the top edge of the tanks inlet. This makes it easier to turn, and less likely for your fingers to end up covered in developer.
There is also a 150mm version in case you want even more elevation (and perhaps second generation compatibility), though I find this to be impractically long for the first generation System 4.

For further convenience, you can print the crank, which will friction fit on top, and give you fingertip control over agitation.

I have not tested this with the second generation (flat top) tanks, so I am not sure if it will cause any compatibility issues, if you have one of these tanks and print this model, please let me know how it works for you.