I built the "Cheapo AC" syst, but it's not working

We built an "exchange chamber" system with ideas from Rod Gervais' book and from the DIY HVAC FAQ here. Instead of using the fan to blow cold air into the studio, we went w/ Rod's idea and used the fan to suck hot air out. Part of the reason we did this is because in this area we don't need AC every day--I've been using this room with just an exhaust fan for 15 years already and only in the summer or when more people are in here than me is the AC needed. Maybe this was a mistake, because even with the AC unit (10k BTU) blasting and the new "exchange chamber" quite chilly there is NO cold air entering the main room. I tried adding a small fan to help the cool air into the wooden duct near the AC unit, but on a warm day (80s inside the room) there was no effect on the inside temp at all. So: bigger exhaust fan? Use the fan to blow cold air into the duct and let the exhaustt be passive (as in the forum FAQ)? Use TWO large cylinder type fans? Any ideas would help--we are stumped. thanks Philip Perkins