I bought four steel pizza pans from my local dollar store, and superglued neodymium magnets to them in a location that centers the cap on my wheels.
I first drilled and then reamed a hole to fit the valve stem through. The wheel rims I have have a central ridge with a diameter of about 7 inches that projects flush with the edge of the rim. The pizza pans have a nice stepped rim that stiffens them and gets the rim 'down' below the airflow.
Locating the magnets is easy. Stick them to the ridge on the rim, put a dollop of glue on each magnet, then put the pan over the whole works. You'll have a couple of seconds to center the rim before the glue grabs.
You can also measure a circle about the right diameter and glue the magnets to the pan before attaching it. I did both, to see what would work better, and saw no real difference.
Now to see if they will hold on at highway speeds and over bumps...
This first step toward aero improvement was inspired by someone's post here about using pizza pans for hubcaps. I didn't have any stock spring-friction fit caps to use, so magnets came to mind. These neodymium magnets are very strong, and I'll find out if they're strong enough.
Total cost- $4.00 US (plus 0.32 tax) and two dead hard drives scavenged for magnets. Total time= about 15 minutes each.