Just a large flat washer with the folowing size.
45 mm o/a dia,
4 mm thick,
11 mm center hole.
The comment by cobraboy is totally relevant, and my sizes are just a guide, making sure that the thickness is of a beefy size and the center hole has clearance.
The drivers side top washer is of the standard type used for shockers, with a dished, concave shape.
I guess that any auto accessories shop or suspension would have them on the shelf ?