Holes in RF shield, what frequencies can escape?
The hole size needs to be &lt;&lt; 10 %    for high impedance mismatch and high return loss.    The transmission loss near /2 and all multiples will be minimum and the leakage might be computed by a function of the ratio of shielded to aperture ratio at those minimum loss wavelengths. I cannot offer a quick formula, however.
Anecdotal
I recall buying the surplus Stanley Door company 120 dB Lindgren Faraday cage from KJ Marketing in Toronto 25 yrs ago and we constructed the double walled copper “bug screen” and beryllium copper fingered doors on the double clamped frame corners in a day in the lab in Winnipeg. It was the size of a large gazebo. It wasn’t exactly -120 dB over the whole range up to 20GHz but perfectly adequate to perform telemetry isolation tests and adjacent channel interference tests free from ambient noise. I designed an optimal integrate and dump RSSI board and the H/W guys built it same day while data collection was automated for graphical results at the same time. This was a design validation test for how many million transmitters can you squeeze in a narrow channel with the smallest guard bands,by measuring the SNR of an optimal receiver (I&amp;D) for both the signal and RSSI level both using integrate and dump detection for the purpose of automated power meter telemetry reading licensed channels owned by power utility companies near 928MHz using TDM, FDM, spacial colouring with pleisiochronous signalling for the AMR business that outperformed any spread spectrum 2 way trial.