All you should need is something like this:
http://cgi.ebay.com/ebaymotors/O2-Se...QQcmdZViewItem
I don't know too much about it; whether it would work on a car with only 1 O2 sensor, but it works if you have 2 (1 being placed after the cat). So you put that on the 2nd O2 sensor. By pushing the O2 sensor further away from the cat, it will fool the ecu into thinking that the cat is working efficiently.