If you don't plan to breed, I wouldn't suggest the use of nest boxes or things that imitate them, like huts, as they can result in excess hormones. This can lead to health and behavior concerns like chronic egg laying, aggression, and biting. However, if breeding is your intention then while the safety of the babies is the priority something like this may be a good idea? https://www.amazon.com/Trixie-Natural-Living-Nesting-12cm/dp/B000WFE5HM
Though it would need to be the right size for 'tiels.
If you're talking about cages (I wasn't sure), I think a natural cage set up can produce a fun and natural look while being escape proof, easy to clean, and safe. This can be done by using natural fiber toys, safe plants, and natural wood branches -- the bark being left on can amplify the look you're going for.