How do I make an air-tight contract to ensure I get paid?

You can’t.  If you don’t trust the person that you are contracting with to pay you, then do not enter into a contract with payment terms with that person.  Insist on cash up front or do not do business with that person.  Your only remedy for a bad faith non-payment is a lawsuit.  And even if you win that lawsuit, you still have to collect.