As a geophysicist, I understand your problem. Pinholes, slit length and width, mesh size, etc will all factor into the suitability for use as a signal attenuator. It comes down to the wavelength of the energy that you're trying to block. Common millimeter or submillimeter wavelengths from ordinary consumer devices make it important to take a hard look at your shielding to identify any stray path opportunities. Transmission and reflection coefficients of the materials and the propagation medium, reflection paths, material density properties, etc. All these things will work with or conspire against you.