I keep coming back to this thread in slight disbelief.
What on earth does it sound like to the audience when you singer repeatedly klonks his teeth on the mic???
Nevermind him worrying about not looking rock and roll by using a wind shield what about sounding absolutely dire?
A few friends, admittedly not long in the tooth, use the pop shield foam on their vocal mics and it looks good, but if your vocalist really can't do it, then I'd do the following:
Replacement sm58 3rd party grille balls can be found in ebay for a couple of quid.
Buy one (or a few)
Buy a pack of foam windshields.
Buy a tube of contact adhesive.
Carefully cut the windshields so they only really cover the top part of the replacement grille (ie the section that gets a good chewing)
Affix the foam Using a couple of judicious blobs of contact adhesive on the outside edge.
This won't last very long, but will be easy to pull off and replace and if you make a couple can be swapped mid gig if necessary.