I dug up and old case fan and wired it up...
That's what I did, Rick (TX-SR805). I found an old power supply that had a smallish plug (rather than a wall-wart) and just plugged it into the switched outlet on the back of the receiver. I have two fans running off that supply - one blowing down into one side of the unit and the other blowing across the top (it is in a semi-enclosed rack). Noise is not an issue.
My understanding is that a good part of the heat problem comes from the video processor, not the amplifier. That's why I have the fan blowing down into the right-rear of the unit (yours may have different architecture).
Both Peter and I have "vintage" Onkyo receivers also. I know mine runs warm. I also know it's been running almost non-stop for about 15 years.