[Christ]

A fairly easy solution would be to replace your studs with longer ones, so you'd have some threads left over to bolt something to.

Another solution would be to use longer lug nuts, so there were some female threads you could stick a bolt into from outside the wheel cover.

There's also the option to clamp the wheel cover on, as opposed to bolting it. You could pretty easily make something from a carriage bolt and an extra piece of tin or coroplast with a wingnut on it. Granted, you'd have to check on it periodically, but not altogether expensive if it were to come loose.