This is a single unit with a captured tail actuator slider.  Just fish the ribbon back through the tail and snap the button into the slider and you're ready to flex it.

I printed this model in PET-G and for a fast print I used a 0.4mm nozzle and the draft 0.3mm layer height (prints in about 35 minutes).

I didn't use any supports and it printed out fine.  if the slider doesn't have enough bed adhesion, then you can fill the bottom of the slider in with a thin layer of plastic.  I had left it open to provide better grip when actuating the tail.

Now you can slide the actuator and the tail will flex.  Watch that it doesn't sting you :)

Sized to fit on both the Mk3 and the smaller Mini (although you may need to rotate the model to print at a diagonal)

I have included the step file in case you wish to add leg articulation or some better contouring

Enjoy!