How's the temperature outside of the grow Box? If it's a moderate temperature being indoors you could get some ducting several feet of it and put the end outside where it's cooler hook that up to the inline fan and bring in that cool air, and open one of your vents. Or buy an additional several feet of duckting and put that on and out going vent and let that hot air out the window.
If you keep your house relatively cold, 70 degrees or so it should make a decent difference.