I have a set of these in my cars:

http://cgi.ebay.com/ebaymotors/New-Mercedes-Benz-W123-240D-300D-300TD-Jack-Hole-Plugs_W0QQitemZ8011830795QQcategoryZ33654QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

They seem to work well.