Shouldn't cost more than £20 for a set of 4 custom spigot rings. Google it & there's quite a few suppliers, plastic will probably be cheaper. Several chaps on Ebay make custom spigot rings. This chap below does them for only £15 for a set of 4 in stainless steel:
http://www.ebay.co.uk/itm/Wheel-spig...item43dff92b5d