I have not heard of anything like that, needing a covering that will reflect a ku beam? Has someone told you the dish can't handle ku band? Adding ku band to existing cband doesn't usually involve doing anything to the actual reflector. You may need to change the feedhorn to one that has a port for both lnb's. One that i know of is the C/Ku Corotor II from Chaparral.
What is your dish made out of? If it has mesh with small holes (that a number two pencil won't go thru) or if its solid aluminum, you only need the c/ku corotor II feedhorn and the two lnbs.