I just had an idea that may be my worst technology idea ever.

Assemble some unstable atoms (that decay into carbon) into the desired cubic structure. When they decay you have a diamond.

The problem with this is that if it can decay fast enough (even with outside neutrons) it will be too hot (pun intended), and if it decays slowly enough it will take too long. Depending on the source isotopes and process it could also result in a radioactive diamond! Also, the heat of the process would have to not change the crystal structure.

However, some day when we master quarks and the weak interaction we might be able to do this quickly and safely.