I'm not bashing anything. I'm just trying to explain a problem with design I see and give you possible solution.
Here is the simple illustration (Sorry don't have any CAD on my PC at work so MS-Paint will do.
a) - regular spacer. All surfaces are parallel. You can tighten bolt nut properly.
b) - your design. top of the arm is not parallel with bottom of the spacer. Not by much about 2.3 degrees, but this still creates a wedge-shaped gap (I exacerbated the angle) under the nut. Yes you can tighten that joint shut but it will only happen via deformation of the bolt. You will have to tighten it enough so it will start stretching on the right side and askew to the left. This is plain no-no for bolt joint.
c) - one possible solution is to cut another retaining ring to go on the bottom of the ball-joint to offset that angle.
I would suggest cutting it from steel though. Another option is what Kevin suggested. Spherical washers. Their working angle askew limit is 3 degrees, your design calls for about 2.3. They will work. It will be probably cheaper solution than cutting new set of retaining rings. Although with tooling already set up for spacers that is just a mirror image. Albeit a thinner one.