I wasn't quite sure where exactly to put this link, because the idea comes up often (so didn't want it 'buried' in a less-trafficked forum), especially when someone's baby has gone missing in the great outdoors. So I'm just plunking it here - you can move it elsewhere, Marc, if you see fit.
Anyhoo, these are some further ideas for building cat enclosures that I happened upon today. Some of them I don't find all that "enriching", but the addition of more plants and other means for shady and interesting areas could easily be added to many of them:
Cat enclosures - Cats of Australia, and
HSUS cat fencing systems list
If I remember right, I think there was also a link in one of Mink & Willow's Mom's threads, too, as well as one other one I'd posted separately somewhere awhile back. Use the "search" feature to find them. (cat fences, or cat enclosures)