Imaging through scattering media, for noble causes such as medical imaging, is a current and productive research area in optics. You can account for essentially arbitrary scattering as long as it doesn't change quickly. While the technology would be different, but I expect that the mathematics and the techniques already exist to thwart and such wall which you might design.
On the other hand, a 5mm sheet of aluminium ought to do the trick.
Three of the British warships sunk in the Falklands war
Physics World has a slightly more in-depth article.
I'd love to, if only I could find somewhere at work where I could to reliably get a 20min nap.
n.b. the work  by Müller, Chu et al is related, but different, and the interpretation is strongly contested (e.g. )
2) As with all the base units, we must 'define' the second in terms of something physical, which we can measure, so that we can use this abstract idea in the real world. This real-world embodiment is imperfect, and it is an engineering challenge to make something which better approximates the idea. For illustration, consider the kilogram, which is defined by a lump of metal in Paris. In principle, chipping a bit off this block makes everything else weigh more in terms of kilograms, but we immediate recognise this as crazy and we can imagine a better physical embodiment of the ideal kilogram (indeed, efforts are under way to do just this). So it is with the second: the caesium clock is the best we've got so far, but it's just a physical embodiment of the ideal second, and we can strive to make a more accurate (with accuracy defined as in (1) above).
Anyone in-the-know care to comment on the relative merits and the relative scales?