Has anyone got a proven recipe to attenuate wi-fi with a Faraday cage?
A common microwave oven is specifically designed to block the frequency band used by wi-fi, bluetooth, cellular data networks, and various other wireless communication protocols. This is because the microwave oven operates at the same frequency band; that's why microwaves can interfere with wifi.

A cheap microwave, or even a non-functional one, is probably the easiest way to get a good Faraday cage of this type. Either put a battery in the microwave with the device, or cut a small hole to feed a power cord through; the hole will cause leakage, but it shouldn't be too much.

As for being a proven recipe... Well, the fact that you don't feel hot standing in front of the microwave is evidence enough, no? If you need more, just stick a cell phone in the microwave and try to call it. It won't ring. (as mentioned above, cell phones and wifi use the same frequency band)