[Kyoshirin]

In the 2D view, you see a trapezoid with a circle inside it. That is a single plate that holds the pivot axle. In the 3D view, you see how it is implemented. With the two plates mounted inside the supports, you can hold up the arm/lever on that pivot axle. In the video you posted, this is accomplished by two pillow blocks (grey omega-shaped things) on top of the pillar. What I am describing for use by you is called a bearing plate, which can be easily made out of sheet metal, a bearing, and three screws.
Does this make sense, or should I try another approach?
Joe G.'s video is another way you could make this automated mallet. How you want to make it is up to you.