PAPR v2

This is an adaption of [my original design](https://www.thingiverse.com/thing:2843547) and [another nice design](https://www.thingiverse.com/thing:3454725).

some pictures [here](https://imgur.com/gallery/oo2sUtg).

This uses:
* [120 mm fan](https://www.amazon.com/gp/product/B01CSNEO2G)
* [Roomba HEPA filters](https://amazon.com/gp/product/B01JWK5CXI) or 3M bayonet filters (both inlet adapters in files)
* Milwaukee M12 battery, the holder for which requires contacts like [these](https://www.amazon.com/gp/product/B01MQ332R6)

I'm including Fusion 360 files for you to modify to your liking. Or, here's the [shared file link](https://a360.co/3epFKmW). I have not yet adapted the design for readily available tubing; it seems like CPAP tubing might work.