Yes, the EU's customs' rules. So let the EU erect and police the border. There is much talk that the UK is "not prepared" to police a border by the time we leave. Since the UK has no wish to see such a border, perhaps the question of readiness should be directed elsewhere.
It seems that the EU is concerned about dodgy goods entering "its" territory from Northern Ireland. Since the UK has no worries about the movement of goods in the other direction I suggest the onus is with the EU to do whatever they think is necessary.
To equate such a "problem" with the UK wanting to regain control of its borders is about as specious as it gets. It wants the restoration of the borders that it once had before the EU decided they were to be opened. It does not want the imposition of one that was never (in living memory) there. It is the EU we are leaving, not the Common Travel Area that is the UK, Ireland, the Isle of Man and the Channel Islands. It is scarcely the UK's fault that Ireland continues to sacrifice its sovereignty to the EU.