I've had these on my watch list for a while Adam - 312mm by 22mm, so no milling required. Potentially could be a good fit along with a caliper mounting bracket?
http://www.ebay.co.uk/itm/180430872840?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
http://www.ebay.co.uk/itm/180430872840?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT