My only fear with strapping through the spokes is that brake calipers frequently come very close to the inside of the wheel. If the wheel or bearing flexes at all in a hard turn, there goes your cover, and if you used metal strapping, you might damage something too. But if you are absolutely sure you have clearance, it is a great idea.
If you have room to strap through the spokes, I would think about using these. You can get them from McMaster-Carr or various other places. I used a bag of them to join 2 umbrella strollers into a double stroller. Make sure to size them so they snugly fit around the wheel spoke. Then if you ever want to remove them, the wheels will not be all scratched up. Use a full threaded bolt, and put the head of the bolt on the side with the sharp bend. Use a bolt long enough to reach past the outside of your wheel cover. Put the clamp around the spoke, and tighten a nut on it to get the clamp down good and tight over the spoke. Then drill a hole in the wheel cover in the appropriate place. You should have at least 3 holes. Another way to go about it would be to use a short bolt and instead of a nut, use a nut coupling. Then instead of having protruding studs with nuts, you could attach the wheel cover with more aero button head screws.
If there is no room to safely attach through spokes, someone from a thread long ago suggested a good idea. Get some lugnuts that are solid on the ends, and drill and tap the ends for bolts. Then drill your wheel covers appropriately. You can use button head bolts to be more aero. Lugnuts are about $1 apiece at the parts store, or you could probably scrounge a used set from the junkyard for almost nothing.
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That holes will be covered with textile universal tape.
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