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.

 

A little anecdote seems appropriate here.
Back in the mid-90s, I was one of the design engineers on a 128 x 128 digital video routing switcher. It contained 128 PLLs, each with a VCO running at 270MHz. These and all the related circuitry (crosspoint switch ICs, etc.) were contained in a steel cube that was approximately 19" on each side. Of course, it had cooling holes, including fans , and lots of seams sealed with lots of screws. We had paid a lot of attention to EMI containment.
We took it to an EMI test facility, along with 256 RG-6 cables of some specified length (can't remember the length), 256 75Ω terminations, a digital video signal source, and possibly some distribution amplifiers. I can't remember the exact setup, but it was prescribed by the FCC or some such agency.
We set it up, powered it on, and watched the technician run his spectrum analyzer, which was attached to an antenna. His first words were "Planes are gonna start crashing.".
We had rolls of adhesive copper tape, and started taping over suspected leaks. We finally got it within spec. Then we had to go back to work and implement the changes in the sheet metal. It wasn't easy, but we finally made a product that we could ship.

not to resurrect the thread into a zombie, but a 128x128 on an airplane in the 90s? What airplane?

 

He didn't say that the equipment had anything to do with airplanes. The EMI test technician simply made a throwaway statement to indicate that the EMI leakage was really bad.