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If your armor is thick/spacious enough, what I've seen other cosplayers do, and what I plan on doing myself, (and this will require some wiring) is using computer fans. If you plan on putting one inside your helmet, grab a 40mm (about $5-$10), a toggle switch (radioshack, also 5-10), a cell battery and wiring. simply connect the wires (gonna need a soldering gun) to the switch, and then the switch to the fan, and ta-da. You can do the same for your torso armor, but I'd go with a larger fan, say 120mm (~$15).With some fans, such as thermaltake's Riing series, you can find them with different colored lights to really elevate your armor.