Replace the cube with a lattice object that's made from a greyscale map you've drawn with the hole included, and made cracks to look like broken glass. Texture that with the mirror tex to reflect your sky. Make everything behind the lattice black, and insert a negative light or two behind it to darken it. Make bits and pieces of broken glass out of lattices as well, just flatten them down to look as thin as possible, and texture those with mirror tex. Either scatter them round the hole and out, or multireplicate them along paths. That's how I would do it anyway

Talent is God's gift to you. Using it is your gift to God.