Sounds like a nice and cheap solution. Not sure how well two co-rotating sets of blades will perform, though I'm sure I've seen some server computer case that used them (noisy as *&#!£$) There are round fans too, which cost quite a bit more, but are designed for ventilation use and a lot cheaper than the bulky bolt-on "marine" vent fans I've seen, one example being the San Ace "RF" (round frame?) series: http://uk.farnell.com/sanyo-denki-sanace-fans/9rf1312p3h001/axial-fan-136mm-12vdc-70-7cfm/dp/2679913 For larger fans, look at combustion engine cooling fans, which are available in surprisingly small sizes, e.g. https://www.ebay.co.uk/itm/153010395167
Edit: For the record, I will be using the Sunon EEC0251B1, of which I have already purchased 10pcs. 184m3/h, or 108 cfm, at 44.5dBA noise level and 0.45A current draw (5.4W). They will be individually controllable through PWM, so can run whisper quiet with very low power draw, while still providing meaningful air flow.