If I am making an object and it is non symmetrical, how can I find a pivot point in it to put the joint centre at?

For example, the picture shows a tube and a thin box, the centre of the box is not the rotation point, how do I find a specific point in the box to put the pivot point?

This happens a lot with mechanical objects I make where it is difficult to find the exact rotation centre so it would be great if anyone has a solution.