I have a small feral colony in my yard, but they're not in an enclosure. They shelter under the house which seems to work well. You might want to consider building the shelter, but allowing them to come and go freely from it. They'll be much happier and less likely to fight. Cats are very territorial, so once you start feeding regularly, they'll make your yard home base and won't roam far...especially once they're 'fixed'. Spaying and neutering is key to maintaining a healthy, stable colony. As far as fleas, I've never had much of a problem. My outside kitties take lots of dust baths, which seems to help. In almost 10 years, I've only had flea issues on my inside cat family a handful of times. Then I use a good back of the neck spot type flea killer (usually Advantage Multi from the vet--expensive but very worth it!), and dust the house liberally with diatomaceous earth. Works great, and I usually only have to do the treatment once. There are also really good organic flea repelling lawn granules available now. I've had good luck with cedar-based products. Lastly, washing your hands after handling stray/feral cats is always a good idea.Good Luck!