Here's the window/balcony box I did within a couple of hours with closet wire shelving and zip ties:
View from inside:
View from outside:
This next one took a little longer and required some help holding pieces in place until tied together, but it was well worth it. The front two shelves can be opened like doors to get from the enclosure into the yard.
The cats absolutely love to sit outside on the grass. And now that they know they can, they are much more relaxed and don't try to force their way out every time I open the door.
(Binky had recently had surgery, hence the shaved neck and scar on the picture above. And yes, those feathers are dead Da-Birds.
)
The best about this system is that it's really easy to put together and/or rearrange or expand on a whim. It's not the prettiest looking enclosure, but the cats clearly don't care.