I know this probably isn't what you want to hear but trapping her is probably the best way to handle this. The main reason being that if she is wild and has her kittens in the wild then you will have all kinds of trouble catching them. If they can be brought up around people and handled then they will have excellent chances of being adopted. If this doesn't happen you will be feeding five more who will soon give 25 more and so on. I hope you can find a rescue organization that will help you trap her.
If not...then I would think any covered and protected shelter will be fine but she will more than likely go somewhere else. She probably already has a spot that nobody knows about.