The idea came from a friend. His original suggestion was that the it be based on the number of adults living in the state - a poll tax. E.g. $10,000 per adult. That would effectively eliminate any possibility of evasion. I objected because it is regressive - poor states would pay a higher percentage of GDP than wealthier states.
But the "poll" tax is the constitutional option.
I.2.iii:
Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers, which shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.