For example, an EU airport could charge max(min(10%, tax rate of country you came from last), tax rate of country you're flying to next).
Thats to prevent an aircraft just filling up where it's cheap and carrying that fuel around the world unnecessarily.
The EU should then lobby other countries to introduce fuel taxes, and if they refuse, set fees for aircraft coming from those countries into the EU which is approximately equal to the taxes due on the fuel for those flights.