Temp control?

I'm growing in a stealth box I built myself. The box is 21" x 37" x 21". The box is completely lined with Mylar and has (6) 4" computer fans for ventilation, 3 intake & 3 outtake. I also had a small stationary fan that I had laying around so I dropped it In the box too. The box has (2) 2' t5 grow lights on the lid of the box. Basically it looks like a chest with 2 lights attached to the inside of the lid. I'm having a problem controlling temp in the box. It sits a 77F in the box. I was able to get the temp down to 72F by putting a small block of ice in the box and having the stationary fan blow on it. This obviously makes the temp & humidity fluctuate because of the melting ice and also the ice needs to get changed out often. Any advice on a better way to control the temp & humidity? I believe the fluctuations will stress the plant , not good.

77 is good... Small fluctuations are also natural. The temperature isn't always the same outside especially during day and night cycles.