I left the hole open for circulation like you were concerned about. It works great, I used to have that "drone" from the exhaust, that is now gone, my wife is very pleased with it, no more headaches from the pipes. I have done the doors and the lid of my trunk also, it is money well spent and I ask myself, "why didn't I do this before?" I still need to pull my seats and carpet up someday to do the floor, I am going to wait til it is cooler out, gets hot in the garage laying it.