Homemade Faraday Cage?
Reading the yearly "this is our conference on EMI for this year", named Interference Control, I recall an article on surviving EMP. The article discussed using ---- yes ---- aluminum foil inside rooms, with TWO LAYERS of foil, overlapped 50%.

The 50% overlap provides a HUGE capacitance between the two layers.

If aluminum-oxide is the dielectric, along with some air, then we can compute the capacitance between 0.25meter by 2meter (1 foot by 8 feet) overlap of two layers of foil, each layer composed of 2 foot by 8 foot pieces running up the wall, across the ceiling, and ON THE FLOOR.

What is that capacitance? Assume air (thickness 1mm thick, even if you use staples every inch to pin the two layers to the cardboard). 0.25 meter by 2 meters

C = Eo * Er * Area/Distance

C = 9e-12 Farad/meter * Air=1 * (0.25 m * 2 m) / 0.001 meter

C = 9e-12 * 0.5 / 0.001 = 9e-12 * 500 = 4,500 pF = 4.5 nanoFarad

Z(C) at 1MHz = 1/omega*C = 220,000,000 / (2*pi*1MHz) = 220,00,000 / 6.3 Million

Z(C,1MHz) = 40 ohms between sheets, if the spacing is 1mm.

If, under the staples, the spacing is down to 100 Angstroms (or whatever thickness is aluminum oxide), then that dominates and the impedance at 1MHz becomes 1mm/100Ang smaller
or 1e-3 / 1e-8 = 100,000X smaller, or about a milliohm between overlapping sheets (50% overlap). Notice the staples are key.

How will you make a door? 

Or have the foil on OUTSIDE of the large cardboard box, and just tape the door over with more foil.