Without an actual cooler your not going to go lower than 5-6 degrees above ambient temp,and thats with 2-4" exhaust and intake(I think were only about 6 miles from the sun down here,When you step out side you can feel your skin start to sizzle)My veg box isn't much bigger than yours and my inside temp gets down to 2-3 degrees above ambient with the doors open and fans running. But add that to 95-98+ degrees in the shade, if your shed is in the sun the inside temps are compairable to that of a car in the sun(130+)Try a solar\120v roof vent to keep shed air moving.I use 2-6" 240cfm duct booster fans from W.D. Armstrong $27.00 ea to move the air thru an 8'x8' room