Why would you want to put a cap on management compensation?
The comp for the highest level of management essentially comes out of the same pool of money as the returns to shareholders. If shareholders want to pay the CEO lots of money, let them. It's their money.
Especially, C-level executive's pay doesn't really come out of the same pot of money as low level employees' pay. The pay for cashiers at Walmart is more dependent on what Target or McDonald's pay their cashiers than on how much money the CEOs make.
Thanks to outsourcing jobs to third parties capping exec pay at some multiple of lowest employee pay doesn't make much sense. Many companies don't directly employ their security guards for example, but pay a third party company.
As for board members: they should be compensated however much they can negotiate for.
If I were to own a company, I would have opinions on how much the board members should be compensated. But who am I to tell other people how much to pay for services?
And I don't think all board members necessarily own significant amount of shares. See also https://en.wikipedia.org/wiki/Independent_director
In any case, I don't really understand how capping compensation would help hold management to account? I can see an argument for how management should mostly be paid in eg stock, so that their incentives are aligned properly. But I don't think that's what you wanted to get at?