A grid structure formed from a plurality of building blocks, the grid structure comprising: a plurality of flat panels, wherein two of the plurality of flat panels are paired in parallel to have one of the two parallel flat panels provide an inner surface to one building block from the plurality of building blocks and the other of the two parallel flat panels provide an inner surface to another building block from the plurality of building blocks, wherein within each of the plurality of building blocks, the inner surfaces of any two adjacent panels lie on planes intersecting along a straight line that passes through an inner corner of the building block.