So 29c is like 84 degrees? That's not terrible, but would be better lower. Even a air cooled light is gonna put off heat. My light adds roughly 7 degrees when everything is running, so if it's 80 degrees in my garage ( sorry don't know celsius) it's 87 in my grow closet. That's why I have to run AC three or four months a year. Everything adds heat, fans, lights, ballasts ect.
You only need so much intake. I have filter/fan combo, not completly sure of the cfm because it was givin to me, but i'm pretty sure it's like 440 cfm. Filter/fan goes to a 6" duct which goes to the light, then on the otherside of the light I stepped down to a 4" duct. Duct blows out of the closet into the vented garage. My passive intake is roughly 5" by 10" cut in the drywall. I started with a smaller hole and felt the pull when opening the door until I thought it was about right. Does your cabinet door kind of close itself from the draw of your fan? I'm just wondering if you have good airflow.
Just remember in a perfect world the temp would be 78 and you would have a 10 to 15 degree drop when the lights went out and the plants would love you, but it's not a perfect world when you have to grow in a small space in secret so just realize that it's great to strive for perfection, sometimes perfection can't be obtained. That being said if your high temp is 84 it will grow just fine. Would I prefer it lower yes but you will still get some of the best smoke you ever had growing your own.
Things you can try: obviously air condition room, exhaust heat from cab. outdoors if possible, because your warming the air thats going right back in the room (ambient temp), run lights at night when temps are cooler and costs are cheaper, get the mechanicals out of the box (hard to do when your trying to be stealthy) feel the heat comming of your ballast alone.
My closet is roughly 4'w 2.5'd 8't and it was 82 degrees in there this morning. Todays forecast has the high temp at 83 degrees. So right now i'm roughly at the same temp. as you. Nothing I can do about it unless I hook the AC back up. I only worry if the heat gets in the 90 range.
Well good luck and hope I was of some help, it's hard to figure out without actually being there.