In case anyone wants more ideas and inspiration, there's a great initiative out of Portland, Oregon between The Feral Cat Coalition of Oregon and The Audubon Society of Portland, where they host an annual "Catio Tour" of people's outdoor cat enclosures. Tons of resources on their website, and if you follow the links, you can check out pics of the variety of enclosure styles. Some as simple as a large dog crate connected to a basement window. Others are huge and elaborate and meant for both cats and people to use. Pretty cool stuff!!
http://feralcats.com/Catio.html