Sounds like you might benefit from a drop trap. I borrowed one to trap a trap-savvy cat. You can prop the trap up and feed them underneath.

There's an attached rope to pull that brings the trap down. I tried it on my patio. I stayed inside the house with the sliding door cracked, holding the rope.

When the cat went underneath to eat, I pulled the rope and down came the trap. Then, I used a transfer trap that opened to a door on the drop trap. The cat ran inside (thinking she was escaping) and then was trapped in the transfer trap. Off to the clinic she went. No more kittens.

The Alley Cat Allies has detailed instructions and video here:

http://www.alleycat.org/NetCommunity/Page.aspx?pid=633

Perhaps you can borrow one from a local feral cat group.

You can also check out the files and links at the Yahoo feral cat group: feral_cats.

Good luck to you! Thank you for taking care of the kitties!