This puzzle cube has 3 unique solutions but is still incredibly perplexing. You can scale the .stl in Makerware to the size of your liking. It is 1.5 inches tall by default.
This was an actual school assignment i had to do.
The criteria for the assignment was to design a 3 cube by 3 cube by 3 cube cube with:
a) 5 pieces that interlock
b) No individual piece can be a rectangular prism
c) Each piece must be comprised of 4 to 6 individual cubes.
I first drew out 30 or so figures on isometric grid paper and chose one that I thought we be the most difficult and unlikely piece to fit in a cube. As I designed each piece of the cube, I made sure that each could fit into at least one more spot on a different part of the cube; this design technique resulted in a very challenging and tricky puzzle cube.