I recommend harnesses for adults. Those are usually easier to train a bird to wear than a suite, unless they were raised and trained to wear a suite prior to weaning.
My bird doesn't like the flight suit either. The harness is way easier for me because he doesn't mind it at all. If i make him wear the hoodie for long or the flight suit, he will start to pick at it.
I love my flight suit. No problems with a dirty vent because I don't keep it on Kiwi for hours and hours. It's only on her when we go out for walks in the park, or for car rides. If her vent does get dirty I just let her splash around in some water after and she's good to go.
As for Kiwi, she doesn't like it and will get really mad at it once it's on, but she gets over it after a while, especially with going outside and lots of yummy treats being a reward.
Putting it on gets easier with practice..i will hold her whole body in my left hand (be very careful NOT to squeeze..imagine your hand a claw cage..firm but not tight!), wrap the flight suit around her with my right, and velcro it on. When it's secured I gently pull her wings out each hole, and then wiggle the butt part and her feet pop out on their own. She puts up a fight sometimes, but if I stay calm and do it quickly, she has no time to throw a tantrum, or even think of one. Just don't make it a big deal, and make sure you reward a lot after, lots of excited talking and fun activities.
Good luck!!
oops..just realized this is for a macaw..sorry! you can't hold a macaw with one hand!