Tha, I have an 805 and the best place to put the fan is over the rear right quarter (where the video processing is taking place) that area generate the most heat. I have a simple 4.00 4" Wally World fan blowing across the top diagonally across the unit and even after two Bluray movies, mine is still very cold to the touch. Best four bucks you can spend on getting rid of the heat.
I also own a Denon 3808 and it gets about just as hot as the Onkyo 805 (so I have a small wally world fan on that one too, and both are in very open cabinets and the Onkyo is in a Bell'o open air rack with 8 inches on top, so heat isnt an issue, but added the fan anyway and it now runs way cooler.