[alister]

> Go get some of the washable 20x30 filters and cardboard, duct tape, and a fan

Here's a Wikipedia reference to one very effective design:

https://en.wikipedia.org/wiki/Corsi%E2%80%93Rosenthal_Box

It's just four 20x20-inch HEPA filters from a hardware store taped together as a cube with a 20x20-inch box fan on top. I remember reading (here on Hacker News) about someone's experimental analysis that this simple design was more effective than most commercial air purifiers.

The only downside is that a typical box fan is quite noisy even at the lowest setting.