We have had several felines who have gradually been trained to be 'harness friendly' as the dangers far outweigh our boys' need for fresh air, particularly in our new home.
In the past, we have tried everything we could find locally, and ended up purchasing a harness meant for small dogs. Marmaduke used to love to have the harness put on for evening walks, and it was stress free for all of us to be able to give our cats some fresh air time when they wanted it, without fear of losing them. One time, Hamlet and I were outside after dark and we walked around the corner of a building right into the path of a coyote. Without the harness, we would definitely have lost him.
A word of warning here in that Legolas learned how to press against the pavement to hold the harness still while he put his front paws above his head and slid out of this well made harness, so we have yet to find the Purrfect harness that will allow him time outdoors.