Heyo,
Me + @dx made a project for #39c3 this year, in which we made 5 DIY air purifiers a la Corsi-Rosenthal Cubes, but modified to be more portable, easier to take apart and put together, and overall be congress-safe. We had them at Angel Restaurant (2), CERT (2) and Kidspace (1), so you may have seen them around.

We have published details and files here if interested: https://github.com/c3infsch/aircycles (feel free to reach out with questions or if you want to build them so we can help)

We just finished taking them apart for the year and threw out most of the filters as they looked really dark, and with that we're done with 39c3 🎉

If you'd like to help offset our costs so far, and help us build more for future events (we got so many more requests than we could supply), we've set up donation links:
https://bunq.me/aveao (also shows the IBAN if you prefer to do a bank transfer)
https://paypal.me/aveaoz
(Transfer reference "Aircycles Donation" or "Aircycles Spende" please.)

As for the amount: We've spent about 700eur not including the costs of prototyping, and for next year we plan to build even more of them to meet the demand. We don't intend to make a profit, so I might edit this post later if donations reach too high an amount (but I doubt we'll get there).

Thank you! (and boosts welcome)