We got one off ebay, handmade by a retired carpenter, it wasn't cheap, but was delivered, fully erected, it's more then big enough for our 3 chucks and has nesting boxes and perches, plenty of access for cleaning and it's really solid. My friend bought one from ebay and it's quite thin and poor quality, I don't think Mr Fox would have much trouble getting in hers. We built our own run, using fox proof wire, dearer, but well worth it, and sinking it 2ft into the ground to prevent the foxes digging underneath.
We use a powder to control mites and other crawling nasties, and we have a felt roof. I've never seen any mites when I clean them out, so it must work!
Recommended sellers on ebay
search.ebay.co.uk/_W0QQfrtsZ0QQsassZchickenfee for powder
myworld.ebay.co.uk/hillsofdevon for wire
myworld.ebay.co.uk/elitetimberhutches07 for the coup - although he has no listings at the moment.