I did it to my pioneer with 3" pvc i actually saw no difference with using it and actually have it discontented at the moment. I think it actually restricted airflow to the fire a bit as you could see the flame increase with it off a little. Maybe 4" would be the way to go for less restriction.
Heres some pics of mine i used a 3" pvc ring and just used the existing screws on the combustion fan to mount it. You can get the ring at lowes/home depot.Finished My Project