I have thought about it, but opted for "no" or a "maybe I'm trying this out in case I would come across a bird cage for free"
Reason why:
Taco tolerates the harness and leash very well. He also sits on me, or on a branch, for a long time, not wandering around. He also won't get more outdoor time with this: I only take him out of the enclosure if he wants, and then I'm at home and have the option to go on the balcony which I do nevertheless and not only for him.
Also, I would never leave him outdoors unsupervised, not even for a short time (like: me going to the bathroom), as of the birds (despite they would not be able to get him), sometimes sudden weird noises (it#s generally rather quiet, but there are both a construction site and a military recinto just across), and we can have high wind gusts from time to time.
So I would not see a benefit in a cage and thus I won't start constructing something that's likely never used.
Generally, however, I think an outdoor enclosure can be a good thing. Myself as well as my in-laws have used, or in the case of my in-laws use, outdoor enclosures for other animals - pets like birds and rabbits, as well as wildlife to foster like birds, hedgehogs, squirrels - in slightly different settings (countryside, in the garden as a slightly covered shed or on the balcony on the first floor). In those settings it was/ is even okay to put the animal(s) in right in the morning and leaving them there unsupervised, or leaving them there slightly supervised (animal in a cage while doing garden work).