Funny you posted this, as I've recently been hunting for the same thing! My birds are even smaller than yours though, which seems to make it even harder (due to bar spacing and gaps).*
Unfortunately I couldn't work out a cost-effective way to supply them with anything big enough for them to fly in that would also be able to withstand our harsh seasons and/or keep from rusting -- without having to drag it in and out of the house all the time which kinda defeats the purpose, in a way.
The conclusion I drew when looking for a solution to this same problem was, it's probably easier and cheaper in the long run to buy something designed for parrots ... if not EXACTLY what you want, then at least something that can be re-designed or repurposed slightly. I've been down many "DIY Outdoor Aviary Idea" rabbit holes already, can you tell?! If you DO come up with something novel please keep us posted, as I'd be interested to see it.
We've just begun considering replacing a portion of our deck with a sunroom (!!!?) as I'm also passionate about keeping lots of tropical plants, in which case, we'd be able to look at installing a free-standing aviary of some design. I'm hopeful that if we keep the sunroom small enough, we can make it happen within our budget. I realize that's not possible for most people, and probably won't be for us either, TBH. But we're going so far as to get a quote to so we can be certain about ruling it out.