I saw a hollow cube quilt in Alicia's Attic booth at the Madison Wisconsin quilt expo a few years back. If you take the finished block and cut it from top to bottom, you can see how it is made without y-seams. You take a triangle and put a frame on two sides. That's it.
I tried to put 3 light fabrics in the center of the blocks so that it looks like there is a light on inside the block. By arranging them correctly, you will see the two "walls" and the "floor" inside the block. On the outside pieces, I put darker fabrics, but arrange them so that you can see the light source; usually the top is lightest, and keep the darkest on the same side; left or right.