Not my idea, just a modified copy.
The hinge point is just a 6mm rod welded to the top of the pendulum and inserted into a 6mm hole at the top of the upright. Friction is not a problem, after mounting, it continues to swing for at least a minute or two, the slightest bump and it starts swinging again. I did toy with the idea of using a bearing or a knife edge hinge, but I think it would take all day for it to stop swinging using the bearing and keeping the pendulum from twisting side to side would require more thought than I was willing to put in. There is a guide rail at the back of the mounting apparatus to ensure it is square to the bed, if not the readings will be out. It also has a quick connect/disconnect system similar to the tailstock mount, minus the cam, although the original I made had a cam lock from a bicycle wheel. This pendulum level is just a copy of the plumb bob level, with a pendulum substituted for the string line, as posted in a previous thread. The math used to work out the discrepancies is the same, coming up at .0185 mm over the length of the bed (600mm). I tested the theory on a rough one I tacked together quickly, it worked so well I made a better one.