Holes in a Faraday Cage

I have observed honecomb mesh being used to combat rf leakage. Probably not cost effective on your small scale. But, some copper tape or mesh would most likely help in the long run. The leakage points in your cage are genrally where the joints meet or where screws are installed that might act like small antennas. Sealing a faraday cage against all leakage could prove very challenging depending on the frequency range. 2.1GHz might prove a bit tough.