A real fan, a 4 inch can-fan for instance and a real hood that is sealed. With that running you might get away with the otherwise useless fans you currently have venting the remainder of the heat.
My suggestion to anyone is that you don't cheap out on hoods, and don't listen to people when they suggest bathroom and computer fans when any wattage of HPS or MH is involved, especially in an enclosed area.
My guess is that a $300 can fan is a little overkill for this guy's grow.
Computer fans, inline vent fans (from Home Depot), muffin fans, oscillating fans, etc...all move air. Yes, the more money you spend the more air you'll probably be able to move using a much more expensive fan. But, if you're growing in a fridge then chances are that it's going to be put together with various parts found here and there.
Nattyhead, don't listen to anyone who says you have to use ANYTHING expensive for your setup. Buy it all used. Rip fans out of junky computers and power supplies. Go to Goodwill and find appliances for a buck that have parts you can use. Look at alternative ways to do this (after all, you're growing in an alternative box).
My box resides in my bedroom closet. It is constructed from leftover plywood and 2x4s. It is 4'w x 2.5'd x 9'h. I have an 8" duct fan at the top of the box (we'll call it a "box") for an exhaust and a 6" duct fan mounted to my enclosed hood. The intake to the hood is open to help exhaust air from the box. The 6" and 8" fans have ducting that join up to a homemade carbon scrubber in the attic for odor control.
I'm using a 600w MH/HPS with my lumatek ballast mounted in the garage (the other side of the wall)
I have my ultrasonic cool mist humidifier ($4.99 at SVP) misting air directly at my 6" oscillating fan ($2.99 at SVP). I have two more oscillating fans higher up for when the plants grow bigger.
I have a 6" duct fan bringing air into the box at the bottom. I need to add another because it's not keeping up with the exhaust.
I cut up a cheap air conditioner filter and attached them to the fans (just used rubber bands to keep it on the fans). This helps keep dust and critters out.
With this setup, my six plant harvest from two weeks ago produced 22oz. I grow in hydro using a homemade 6 x 5-gallon bucket + 30 gallon sterlite reservoir DWC accompanied by 15 min incremental on/off sprayers. I got the 15 min mechanical timer from Home Depot online for $4.88. I had to search around for this because there was no way I was going to spend $80 on one of those hydroponic specific incremental timer that does the same thing. Home Depot doesn't sell these in their stores so you have to get them online.
Point is to try things with stuff you have around before you start looking at expensive solutions. Chances are if you get creative enough you can solve your issue and have some cash left over to buy a vaporizer or something
Good luck