What about setting up a sort of "vestibule" with baby gates or plywood or something like that around the door (inside or out), so that there isn't that direct access to the outside. The humans can step over it or slide it open and it creates just that little extra delay to prevent your cat from making a run for it.
"To close your eyes will not ease another's pain." ~ Chinese Proverb
“We must not refuse to see with our eyes what they must endure with their bodies.” ~ Gretchen Wyler