Basically a cup is created for 64 teams of closest quality starting from the top of the server. (Top 14 players of them)
So best 64 teams of the server, then next best 64, next and so on. If you want lower opponents, the only thing you can do is lower your own quality and need maybe some luck.
In my high level, I have for example a lot of -1 opponents, also some -3 opponents in CL for example.