Kirby has an aviator harness that works well for him. He definitely prefers it over a flightsuit. It has always been secure and comfortable for him; I've been using his for two years, I think, and it's been great. I don't remember what size we ordered, but I think they have a sizing guide on the website. Just make sure to carefully desensitize the bird to it before using it. Don't put it on right away, make sure to just get to the point where having it in his presence is a good thing, then build to being able to touch him with it/pet him with it wrapped around your fingers. That's when I'd start putting it on, and even then very slowly (i.e. maybe you spend a couple of weeks just putting it on his head) using baby steps (i.e. putting it on his head can be broken down further - how far down depends on the tiel) and making sure that it remains positive and happy all throughout. It will take a long time, but trust me - better to take your time laying down a positive, strong foundation than to have to go back and fix a rushed mistake later.