Do it yourself drop trap:

http://www.droptrapdesign.blogspot.com/

Please be careful--a too-heavy trap can injure a cat or worse for kittens. The above link tells you what you need to know.

However, you do need the transfer trap to mate to the opening in the drop trap. There's a picture of this on the above web site.

BTW, the people in the feral cat groups swear that Kentucky Fried Chicken, deboned, is the very best cat bait if you want to try that in your trap.

Good luck!