Harry Bloomfield wrote:

A ball joint would not be rigid enough, the joint angle would
slip with the lightest pressure. What is needed is one of those
joints where it locks with teeth giving a choice of several fixed
angles. That would need much less pressure on the joint bolt to
make it stay where put.

As used on parasols over garden tables? I'll bet a pub would have a few
old ones which could be cannibalised.