If the majority write it [the natural law of a constitution], it can't protect the minority. And if the minority write it, it's not democratic.
I am not sure what you mean by 'the natural law of a constitution'. A constitution defines the powers and structure of government and the legal rights and obligations of citizens. If there is such a thing as natural law, an idea defended by a long tradition of political philosophy and ethics, a constitution might embody it or some portion of it. The constitution would then involve assumptions about the rights and obligations we have, given the kind of beings we are. There is more to natural law than this but there is at least this. Natural law may be seen as a deliverable of and discoverable by reason or as having a divine origin - or both. Its existence is controversial.
But I venture to suggest that you are not using 'natural law' in this sense at all. My impression is that in your view it is inherent in the nature of a democratic constitution - 'natural' to it - to protect the minority (or minorities) from the majority. You then pose the dilemma about who should write the democratic constitution, who should create it : the majority or the minority?
Is there really a problem here, a philosophical or conceptual problem? Here's why I think not.
A constitution is democratic by virtue of its contents and implementation, not by virtue of who created it. There is no reason why a majority should not create a constitution that protects the rights of minorities. And there is equally no reason why a minority should not create a constitution that protects the rights of the majority. Nor is there any reason why a constitution cannot both protect the rights of the majority, within prescribed limits, and protect the rights of minorities also within prescribed limits. There is no inherent incompatibility between the equal protection of rights for the majority and for minorities within a constitution.