What is the best way to organize boxed versions of a product?

Question

Software design

Hello. I am a cloud service developer. My clients can buy a boxed version of the service from me with their modifications and features, depending on the specifics of the company itself. From the very beginning, I did not foresee the branching of the project into boxes and did not develop a modular architecture, but I have 3-4 clients, each of them has their own Wishlist according to the system (an agreement with them has already been drawn up). Now I am faced with such a problem, for example, I found a bug in the core, I fixed this bug, then copy-paste this bugfix to various boxes, but something tells me that this cannot be done, can it be done at the level of git branches?
PS: A project on MeteorJS, after a while I will start re-architecting the system and make it fully modular, while a temporary competent solution is needed, so to speak, for six months.