Perhaps you could add a clause “if you don’t fly the slot, then you pay most of what you would have spent on the flight”—high enough to ruin the perverse incent

Perhaps you could add a clause “if you don’t fly the slot, then you pay most of what you would have spent on the flight”—high enough to ruin the perverse incentive, but low enough to encourage not making the flight unnecessarily. And perhaps an arrangement that most of the surplus goes to some charity or other.