I guess I'm confused. Do you want something for them to be
on the patio, or do you want something for them to be
outdoors, like and outdoor area on the grass? Because if you want an outdoor area, I know someone who has used this cat fence...
http://www.deerbusters.com/cat---dog...g-systems.html They have an acre fenced in with this for their feral cats, and it works.
Check out the two patio cat doors on this page. Last two on the right...
http://www.moorepet-petdoors.com/Cat...FQudnAodTVWQ0g
http://www.hitecpet.com/copetdo.html more patio doors
Maybe you could contact these people.
http://www.thecatshouse.com/ I know they are doing consulting work now, and they had been designing a way for their cats to be outside in an enclosure. (I really wish they would get the site up and running again. Their house is an amazing playground for kitties.)