I would also like some info on this, I have heard back-pressure on the fans will shorten their life span. I don't know how that specifically applies here, I suppose it means that you need enough surface area within the carbon filter to accommodate the CFM that the fan is pushing/pulling, I just don't know whether pushing/pulling makes a difference. In my setup an internal scrubber would be better for space reasons, however I have not seen anyone with such a setup except big grow rooms. I also need a way to calculate the size of filter needed in a DIY rig based on my fans exhaust speed of roughly 40 CFM. I could calculate it based on larger commercial setups and scale it down, but I haven't tried the math and have no idea if its a linear calculation.
This is the coolest DIY scrubber idea I have seen so far. I saw another one that was larger and done with a bucket, some screen and 4" ducting, but that would be for a larger setup.