The antenna you link to appears to be directional (9dBi) already. If you can figure out where the main lobe of the antenna points (the specifications didn't seem to specify), you may simply try to aim more accurately.
An effective metal structure, in the manner I think you suggest, would probably be a fair-size satellite dish. 2.4GHz is around 12cm wavelength, so a dish would need to be several multiples of that, perhaps a meter or so diameter if I remember well enough. Anything smaller would be subject to diffraction and the radiation would just bend around the metal, perhaps even just reducing the overall radiated power due to losses.
Other modifications would require you to accurately space some sort of parasitic element to accurately re-radiate with the right phase. (Think of an outdoor TV antenna, or Yagi antenna) It may be easier to buy another antenna or build from scratch as you need to know more details about the antenna you already have, like its input impedance and radiation pattern, which changes when you put stuff around it.