The metal slide is probably 5 x 10cm, and moves with little effort.
Yes, I had thought of that also. Like a steam locomotive drive wheel and piston. However the whole reason I started this project was to introduce some precision to the amount the metal slide opened. If I try and fabricate something it'll be a lot of work and probably less precision than if I just buy something from Amazon.
I can easily move it in and out with my smallest finger, although with fertilizer loaded there might be more friction.
My first thought was to use someone's 3d print in conjunction with a continuous servo. However I've had trouble controlling the continuous servo. It needs to rotate a certain amount in a certain direction and then stop
Do I need any linear actuator? Do I need one with a potentiometer for position feedback? Or do I need a linear servo? Is there even a difference?