How Can I Stop My Xbox 360 Overheating?

I have looked all over the internet for a solution, and ive tried all the practical ones. It is stood upright, the back is directed toward an open window, the wires are straight, the power block is far away from the console, and it is well ventilated. Despite all this, all it can manage is 20-25 minutes of gameplay, and then the 2 red lights flash. After that, i need to turn it off and wait a couple of hours for another 20 minutes! If i dont wait this 2 hours, i can only get like 3 minutes play from the damn thing. Does anyone have a cheap, easy, DIY solution i could try? Much obliged.