I don't think there is any problem with this as a thought experiment. The container does not have to be infinite and when you close-off the container it will contain a radiation field with a finite energy density. 

However in practice, even the best reflectors have a finite conductivity and a less than perfect reflectivity, so the radiation field would dissipate by heating the walls of the container.

You could also think about construcing your container out of a 
solid dielectric block, such that the light was totally internally reflected at each interface (an optical fibre). But again, there is no perfect dielectric and no perfectly smooth surface for ideal specular reflection. Eventually there will be absorption, scattering and even losses to the exterior through evanescent wave coupling.

Capturing a light beam
I have used an electric shower for years no problems, but all of a sudden my fuse board started buzzing when I use the shower. I replaced the shower with the same size unit and it still makes a buzzing noise.

I have checked all the wires and they are tight and well connected. I changed the old cartridge fuse for an mcb 40amp fuse, and still it makes a buzzing noise.

Is this a sign that my wylex fuse board needs changing?
