I'd set the trap in the area where you've seen her the most with sardines. Cover the bottom of the trap with leaves and stuff, and cover the trap itself with an old blanket or towel so she won't see it's a trap. Dribble a little juice and a small piece or two of sardine in a trail to the trap. Not too much so that she'll fill up. If she's as skinny as you say, she'll go right into the trap and devour the rest of the sardines. (cut a small paper plate into the shape of a piece of pie). Put it (with the sardines on it) in just near the trip to the trap (not all the way back because she may try to grab the fish with her paw. Once you catch her, throw something over the trap, cuz if she's feral, she'll thrash around and act like a crazy cat. Covering it will calm her down.

Good luck and keep us posted.