Divide a cube into 27 smaller cubes, 3x3x3. Consider subsets of the smaller cubes which are visually distinct (modulo rotation) when viewed from the outside: essentially the presence of the center cube only matters when one of the center face cubes is missing, providing a window.
Perhaps only consider rotations around the vertical axis. The bottom face is attached to the ground and cannot provide a window to the center cube.
Add constraints that the cubes must be connected through faces. Or that they must be stable. The 4 corner cubes on the bottom face must be present.
Subsets of edges is better: there is no obstruction by faces hiding things behind them.