Hey Robert I made one from a fan from a computer which was only 3" and I made the thing longer so I could do 2 packs or add something to help cool more, it does make a difference, so at the field today my packs in the shade before flying was 93 degrees and I really wanted them lower so I took a reusable cold pack like for sports injuries, stuck it in the incoming end and the batteries in the out going airflow, this really brought the packs down to a 80 degrees which I felt was better for a really hot day. I have always charged my packs straight out of the plane without cooling and never had a problem till last week when I used a pack that was 114 degrees off the charger not because of the charging but because they was 145 degrees when I started the charger, charging doesn't put much heat to a pack charged at even up to 3C which I do with smaller packs all the time but the 3850's I can only charge at 5 amps about 1 1/3 C, all this bought the cold pack up to 78 degrees real fats and cooled the batteries very nice in about 10-15 minutes, so the next time I tried a water bottle out of the freg and that helped also. I tried wrapping the packs with the cold packs and that helps but not near as much as the blowing air and the cold source. Next I'll get the bigger fan and build the same where both ends can be used.
Thanks for a great idea for these hot summer days.
Steve Maxwell
PS I used my power source or the deep cycle battery for power.