The owner of the local apple orchard has agreed to let my kids enclose some baby apples in acrylic cubes early this season, so that they’ll grow constrained within the box into a cube shape. If anyone else wants to make rubik’s apples, here’s the design. It’s sized for a 2.5" interior, but it’s parametric so if you can modify the LWH parameters in Fusion 360 to make it any size you want. The F3D file is here.
Basically you just assemble the box around the apple, sliding the stem through the slot in the top, and secure the ends with the friction pins. There are 4 holes in the top corners to add string/wire to support the box from the branch. Don’t forget to engrave your name (in reverse), favorite Star Wars symbol, or Rubik’s lines on the interior surfaces so the apple will become embossed with it.
Thanks to @eljefe4 for the honeycomb bed hold down pin design shared here earlier, which also works perfect for this use with no modification! I put some in the SVG so you don’t have to hunt for them.
<edited 2018-06-15 to include drain holes>
<edited 2019-09-18 to provide SVG for 2" square interior box>