How can I render my 2.4GHz antenna directional?
Since the wavelengthn is quite short (~10 cm) it's very important that the reflector (that can be a can, as proposed) is placed carefully, in a way that the reflected signal is as much as possible in phase with the directed. You achieve this by having the reflecting surfaces at a distance that is multiple to half a wavelength.

You can try to tune it by measuring the signal strength while varying the reflector position, but I wouldn't expect great improvements unless you use a specifically made antenna.