Christina:

Your intent may be admirable, but I fear you are unlikely to have much success. Building a "shelter" will entail a significant amount of work, which will inevitably leave a fair amount of scent. The fox will likely avoid your shelter like the plague.

No one likes to see sick wildlife. There is very little you can do. Sometime our efforts at providing "assistance" are often counterproductive. If you attempt shelter, feed or otherwise make life easy, you'll often drive animals off. In other cases, when you've been 'successful' you may actually encoruage other foxes, coyotes, dogs and other mammals to frequent your area, thereby increasing their chances of contracting what disease or ailment the fox suffers from.

It most cases it seems like the best we can do it leave these creatures be. if the ailment or disease is serious it sometimes seems more humane to end suffering quickly. I've long believed a well placed bullet is better than a long, lingering death.