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Old 5th February 2012
Well, there is no definitive answer to 'good enough'. It all depends on how thick you are willing to make them. So if you can double them up and go 6", yes, that will be better than 1 batt thick at 3". Also, the knowledgeable folks around here say to create an air gap behind the trap. An airgap equal to the depth of the trap almost doubles the trap's effectiveness. So if you want to hit a homerun, create a 6" trap with 6" airgap. I built mine on top of the frame as in this video, unlike the video you linked to, where he builds it into the frame. There are two advantages to building it on top rather than in: 1.) you are automatically increasing the airgap behind it by the depth of the frame and 2.) you are blocking slightly less of insulation on the sides. The one downside to this method is that the trap looks more pillow-like rather than nicely squared off.

The three I've built are 3" deep (1 batt), and about two inches off the wall (the frame distance plus an additional 1" spacer.