Sorry, doesn't work. Light still exerts pressure, so even if you could losslessly trap light in a storage container indefinitely, the container would explode lo

Sorry, doesn't work. Light still exerts pressure, so even if you could losslessly trap light in a storage container indefinitely, the container would explode long before the energy density became 'infinite'. Furthermore, the energy and momentum in light contributes to the gravitational field, so even if you could make an infinitely strong mirror-box, pumping too much energy into it would eventually result in collapsing into a black hole.