This is what we got the cats when we moved house if anyone is interested in building their own.

We bought it off the shelf: the poles are aluminium and the netting is apparently UV-resistant, super dooper netting from Germany (I never knew they made the best cat netting in the world ). It's about 5 metres long x 2.5 metres wide x 1.8 metres high. It's pretty light so it can be moved around or from house to house. There is a zip in the side for hoomans to come in and out - to attach it to the house we cut a small section, stretched a length of washing basket horizontally in it and stuck it to the cat door.

If you want to make your own, I'm sure you could use timber for the frame and maybe shadecloth or some other netting to keep the darlings in

It has a hammock for sleeping, a house to sleep and play in, and a scratching post with steps to get into the hammock. And seeing as we have no garden at this place, their pen got our pot plants as landscaping!