with upto 4 people carried, I've never had a problem provided the pollen filter is in good condition.... removing it will give much greater airflow but the wife has severe hayfever and I like to keep breathing.
in my old tranny, I got 2 cheap plug in fans (5 inch fans) and wired them together... NOT pos to pos, neg to neg but plug to pos, this fans neg to other fans pos, 2nd fan neg to 1st fans plug neg (hope that makes sense)... so the power is daisy chained and the 2 fans work as one but at slow speed.
the fans were in the middle and pointing at the corners of the screen.... worked a treat, even in long traffic jams..... I've just not had a problem with the 638
oh, and the reason for the double wiring..... they were too bloody noisy at full speed.
a couple of compter fans would be ideal.... they are 12v and pretty quiet.