Do you have any cites for the rigidity assertion? (Seems like they're logically separate questions; it's not obvious how malleable non-genetic contributors would be.)
I have kept an eye out for Conscientiousness interventions for the past 2 or 3 years, searched on occasion, and have seen nothing ever suggested except possibly stimulants like amphetamines. Possible instances like military experience generally turn out to be selection effects. As well, the Duckworth paper linked in the sibling comment, extensively discusses the numerous correlates and versions of Conscientiousness before adulthood stretching all the way back to early infancy.
So, as much as I'd like there to be some easy environmental intervention to boost...
If it's worth saying, but not worth its own post, even in Discussion, it goes here.