hmm... my temps run pretty high. CO2 helps under high heat. I could be wrong but when i was running DIY CO2 (yeast/sugar/water) i recall it dropping my temps like 5 degrees. also i have a false back, just the thin cardboard backing with the reflective material. having the false back lets air creep in and out at its own leisure.
i also had an idea the other day. if your cabinet is really similar to mine, the floor of the cabinet interior is actually raised a few inches above the ground and there is nothing but open space on the bottom. i've considered 3 1-1 1/2" inch holes (using a hole saw) for passive air intake, and possibly extra holes at the top on the back for more air-out. also PC fans in the bottom (same spots where the three passive holes would have been) combined a bigger central fan up top.
just think about air and how to get it in and out. hot air rises so to bring in cooler air, more holes at the bottom, and less at the top to create more of a suction effect for the warmer air escaping.
ohhh AND!! if you end up using a light shelf like i did for your hood don't forget holes in the shelf to get that hot air up and away from the lights.
i think everything i've said is right. think about it, give it a go. and let me know what you end up doing.
hope this is helpful!!!
Shack
PS i recently pulled the plug on the grow running in this cabinet. going to rethink things, and start over with two soil pots. might configure a shelf as a screen to SCRoG this puppy!!