The Reconstruction Finance Corporation (RFC) was established during the Hoover administration with the primary objective of providing liquidity to, and restoring confidence in the banking system. The banking system experienced extensive pressure during the economic contraction of 1929-1933.
Typical materials used for electromagnetic shielding include sheet metal, metal screen, and metal foam. Any holes in the shield or mesh must be significantly smaller than the wavelength of the radiation that is being kept out, or the enclosure will not effectively approximate an unbroken conducting surface.
In theory, there could be smaller waves but currently we can't detect them. Also, something has to transmit the wave in the first place, so gamma radiation is emitted when particles go through radioactive decay. On the large scale, theoretically electromagnetic waves could be colossal, the size of the universe. But in practise, again you need something to transmit the wave.