A magnetic flea is a rod about 30-40mm long which is placed at the bottom of a container to mix fluids.  The flea is magnetic and is rotated by an external magnet.

A placed a small neodynium magnet on each end of an 8mm steel rod to make a bar magnet.  This is enclosed in a 3D printed plastic case with central ridge to facilitate rotation.    I glued the two halves of the case and covered it in expoxy because some of the chemicals  in the retort are noxious and the 3d print seemed slightly porous.  The flea is dropped in the bottom of the retort.

 An openscad sketch of the case is provided.  It would be easy enough to make a non-3d printed version.

The motor is a geared robotic wheel motor held down by a zip tie.  I cut a steel rod to the same size as the one in my flea and put a magnet on each end then drilled a small hole in the centre which I glued over the axle of the motor.  This made a rotating bar magnet which I placed under my retort.

Testing of the system was successful.