The trick currently used with the invisible tanks is different to the technique that would be exploited using the "invisibility cloak" type approach mentioned.
Invisible tanks use a projection system to project the view from one side of the tank on to the other side, so that an observer thinks they are just seeing the normal view. The new materials being developed at Imperial actually channel light around an object. This is alluded to in the bottom of the article you mention buttercup, but the article does not state what the metamaterial is made of - I haven't seen any papers on this metamaterial myself but they exist and may mention how the metamaterial is formed.
