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I printed this out in PLA with 15% honeycomb (2d) infill and no top or bottom layers for maximum air flow. I attached the fan with two 3mm screws and nuts (the fan I chose has nut traps) and attached the battery pack with a fat rubber band from a bunch of kale from the super market. Bonus: kale is good for you.

It cools the phone much more effectively with the back cover of the phone off of course.

The battery pack adds some decent weight to what is already a pretty face-heavy apparatus, so you'll almost definitely want to use the goofy over-the-top-of-the-head strap with this alternative cooling cover installed.

The OpenSCAD file does have some parameters that can be meaningfully tweaked, but it's not truly parametric with regards to most of the fundamental geometry and dimensions. Play with it.

3-8-15 Added a second version which better centers/supports the battery pack and blocks the fan from blowing directly on your face.