The easiest way I've found:
I made smooth coroplast wheel covers and secured them with zipties going around behind the spokes of the rims. Just cut a slit or make two holes in the wheel cover, then wrap the zipties around behind the spoke..repeat 4 or 5 times for one wheel. If you need to remove them, the zipties need to be cut, thus making it not easily attachable again.
OR
You could tap the ends of lug nuts for something to screw into and attach the covers that way.
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