Dang ... I wouldn't put any external fan for my iMac, it beats the purpose of iMac being fast and silent computer. You're getting same experience as custom built PC if you add external fan, I hope you didn't add any colorful LED and blinks to it 😛
But if you insist to do it, the best way to "help" iMac cooling is put some fans below your Mac screen to blow some air, yes .. close to air intake holes, and add another fan above the screen with sucking mechanism. That would look ugly, but if you really want to circulate air quicker, that's the way it's done