The total cost of this project was under $50 (not including the kitty door which was already installed.
The 20" x 36" x 72" "cage" allows our cats to step outside and observe the critter highway in back of our house
The tricky part of this project was that the whole thing had to be secure, but the sliding screen door behind it had to still function.
Since kitties love high places and afternoon sun, I also built in three sunning shelves for them to lounge around on. The "front" or large side of the cage is hinged and has two barrel bolt latches. The entire panel can be opened for access. The primary materials are 2 x 6 wire fencing and redwood 2 x 2s.
I only had to construct two sides and a top. The top is a piece of plywood which also acts as a storage shelf. I also added a spring to the hinged side as safety measure, in case one of us need to get into the cage for some reason, and stepped away from it, leaving the side panel open by mistake.
The spring closes the door so if a forgetful human opens it up and leaves without latching it, the door will close by itself. It's up to the human to remember to latch the bolts. The theory is, that if the door is just closed but not latched, a cat who ventures out into the cage will see that there is no opening, and not try to get out. The barrel bolts prevent cats who try to get out from escaping.