Cube empty inside

[Witchking]

Is it possible to make cube empty inside other way than using 6 planes, could you give simple example? I've built cube with bouncing balls inside using 6 planes but I've got some problem with this, after hiting the corner ball loose velocity and I don't know why it starts to move in 2d (in one of the dirrection velocity is 0). It's really strange. I think it could be connected with the way I build my cube.

[Flix]

Try using six box shaped walls...