There'd been a thread here a month or two ago about axial fan units with dual blades, rotating in opposite directions.
I just ran across some of these (probably for Dell) at american science and surplus for $3.95 each
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At 1.5"x2" they're probably too small to be immediately useful, but it might be interesting to have one or two in hand just to check 'em out - and $3.95 isn't going to break the bank...