I believe that the squirrel cage fans pick up air at the center of the circular part, then obviously blow it out the straight part. The computer fans as shown above look like they circulate whatever air is in the helmet, unless they are vented from behind somehow. Any air circulation is a plus. My local Vader has zero fans, I can't figure out how he survives ! That's a really nice, cleanly designed bucket interior you have Slinger !!
Another option, which I haven't really seen anyone do, is to mount a fan over the keyslots in the back of the helmet. On one hand, you give up accuracy by losing the calculator piece normally placed there, on the other, it's a sweet spot to ventilate ( provided your hair doesn't get caught ! ).
I also thought about getting speaker grill cloth or some other thin material to cover the keyslots. You could paint it or silkscreen it to look like the calc. part, but have a fan there to blow air out, or draw it in.