It's not possible to store a BIP 39 mnemonic HD key on a normal Rubik's Cube.
But it's possible on a 4x4x4 Rubik's Revenge Cube.
It can be done in 19.36 seconds if your name is Feliks Zemdegs (world record holder).
The rest of us will probably stick with
Cryptosteel.
And yes: I'm thinking about mechanical ways to store your seed that is simpler to use than Cryptosteel, allthough I love their product. It takes like half an hour to an hour to prepare it, but you only have to do it once in your life.
Bitcoin Cash has brought back the creative side of me in bitcoin. Bitcoin is back!
Another similar project i did 3 years ago was a way to create your BIP 39 seed with 4 special Dungeons and Dragons-dice in a plastic box with internal walls. Three D8 dice and one D4.
In addition you need this pdf I made with the table that maps the dice combo to one of the 2048 words in BIP 39.
http://shludvigsen.com/dice/DiceList.pdf
And you also need this BIP 39 checksum tool a cool guy named Liam Lang made for me.
https://gist.github.com/LiamLang/5282c62808048856bfe6
Put your tinfoil hat on, use a computer with all Bluetooth and WIFI antennas ripped out and run the checksum tool before you crush the computer with a sledge hammer and distribute the broken parts on different places.
This is the safest way in the world to create a BIP 39 HD wallet. Never trust RNG's