I'm currently using a homemade metallic spaghetti strainer with a wifi adapter inside it to increase my wifi adapter's signal reception, but would think that a different material, like the aluminum mesh used for the Arecibo Radio Telescope could be better?
The WiFi frequency range is pretty much the same as a standard microwave oven. The reflective coefficient of the metal mesh in the front door is above 99% and it contains no special materials.
On the other hand the shape of the reflector has a large influence. So either you can follow Martin's suggestion of a can antenna, which is widely used or buy a cheap satellite dish. Even a small 40cm dish should boost your signal by several orders of magnitude. Both ways only provide a narrowly focused signal cone so you have to optimize the alignment carefully.
Wifi is around 2.4GHz so a wavelength of around 12.5cm Anything metal with holes much smaller than 12cm will work - even a mesh.
But if you don't have a parabolic dish, only a spherical one, you might do better with a can antennea see eg http://www.turnpoint.net/wireless/cantennahowto.html for details of the calculation