This is a cool test and a great use of (relatively cheap) rtlsdr dongles. Ignoring the "prepper" aspect a faraday cage is genuinely useful thing to have around if you want to play with radio transmitting without any chance of breaking any laws.

A much better faraday cage is matroiska doll nesting of two different size trash cans with a non-conductive platform supporting inside smaller can. The air gap and double conducting surfaces really help. And with the different circumference of the smaller can's rim there'll be a different resonant freq that gets let in. So while the outside seam may pass ~339 MHz without any attenuation the inner seam might pass ~450 MHz and so both unintentional passbands will get at least the minimum ~20dB attenuation per can.