If they required these managers to invest significant amounts of their own money in the company before taking the job, they wouldn't take the job in the first place because it would always be a horrible investment from a diversification standpoint.
The simple and easy solution to this is to completely eliminate the shielding that exists between investors' personal assets and the concrete results of corporate policies.
If an investor is personally responsible for everything his money helped cause, then extreme diversification becomes a severe liability. Did you invest in an investment fund that purchased shares in your name from a company that was found to subcontract to a company involved in child slavery? Now you are guilty of personally trading in child slavery, will all the legal and criminal implications involved in it, including the legal obligation to fully compensate your victims, no workarounds allowed, not even insurance shielding. Don't want risk ending bankrupt and in jail because you put your money where it shouldn't be? Don't invest in anything you don't know perfectly well, be always on top of things, and never allow it to lead to anything even remotely socially negative.
That alone would do wonders in making the entire world of big business turn from the greedy rapaciousness we came to expect of it into one in which every rich person takes extreme care about where they put their money, as putting their money anywhere, including in a bank account, now has consequences that go far beyond merely losing that amount.
Investment funds would still exist, but they'd be very different. Instead of just huge analytical schemes trying to find where's the highest ROI and switching what they according this one single parameter, they'd have troops of managers directly combing very finely every legal and moral aspect of whatever they were looking into putting their clients' money so as to be absolutely sure not only their clients, but they themselves, would not end up poor and in jail.
Do you know that old saying according which freedom requires responsibility? Modern capitalism has set things in such a way that the rich have tons of freedom and very little actual responsibility. Fixing that doesn't require toppling capitalism. Rebalancing things so that responsibility grows proportionally with freedom would to the trick.