Here are nine polycubes with diagonal symmetry. Each piece is obtained from the previous piece by removing one cell.
Put the pieces together to form a cube with a hole inside. The solution is unique.
Or put the pieces together to form a 7×3×3 rectangular block. There are many solutions.
The Kaleidoscope Cube Puzzle was inspired by László Molnár's IIVII Puzzle and his Void! IIVII has six nested diagonally symmetric pieces that form a 3×3×3 cube. Void! has nine nested pieces, not all symmetric, that form a 4×4×4 cube with a hole.
You can buy copies of the Kaleidoscope Cube from Wood Wonders.