So your test may seem successful, but the box not be enough closer to a tower, or if your neighbor added a new AP, or if cell phone frequencies changed since you bought and tested it. Moreover, if you have to open and close it often, you have to be careful that the attenuation doesn't degrade.

All that to say that the "Faraday box solution" would mean testing it regularly with professional instruments and all that. It is far simpler and cheaper to ask guests to leave their phones in their cars.