I’ve been fiddling with the designs for the 3x2x1 Rubik’s Cube incorporating some of the changes I had considered:

• I’ve widened the holes for the nut and bolt.
• I’ve made all of the cubes solid.  I think it will just work better with solid parts.  I had left the four end cubes mostly hollow to:
  • Improve printing time
  • Use less plastic
  • Possibly make an interesting hiding place for something very small
• I’ve shortened the “stem” that connects the semi-circular tab to the end cube to allow for a more snug fit.
• I’ve made the semi-circular groove in the center cubes narrower, to allow for a more snug fit.
• Make these cubes solid so that they slide against each other easier.
• I’ve noticed that the nut and bolt I installed into either side of the center cubes were pushed sufficiently well inside that they don’t want to come out or rotate.  This is a very good thing for this design!  It turns out that if you rotate it too much in one direction, you can just rotate it back a few times and it will be tightened back up!  It also makes for easy disassembly/reassembly – just rotate the puzzle until it comes apart and rotate it back together to reassemble.