Yeah I thought about those too, but with it wrapping around the front of the wheel to decrease the drag you get from a rotating wheel.
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You could make a set that’s two pieces that have a uniball effect so there is no bind on the seat. Commend the amount of work to design and 3d print seat spacers 
They do make some spherical like washers that do allow for some misalignment. I can probably print something similar.
Luckily I’m quite good at CAD. It probably took sub 5 minutes to draw and hit print. Within an hour I had parts ready to go.
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Here you go in M8 McMaster-Carr