DIY Time 
I was skeptical, at first, about how much a DIY filter could do, but there have been some engineering studies that won me over.
Our first household improvised air filter is a 20"x20" pollen-grade filter bungy-corded to a box fan. Not perfect, a bit noisy, but judging by the filter colour every couple of months it’s doing plenty. We had an idea for a variant that uses a Honeywell fan that’s not only fairly cheap, but already kicking around the house.
Today’s project used a 16"x25" filter on one side of the triangle, two corrugated plastic panels, cut to match the filter, for the other two sides. Equilateral triangles top and bottom. (The plastic is cheap enough at a craft store, or use your favourite politician’s old lawn sign). Duck tape the seams on the inside, clear tape outside. The fan comes off its original stand by gently lifting off the side caps and undoing a few screws (save the parts). Cut a tidy circle in the top with a compass and an approxo-knife. Thread the power cable of the fan through a corner, and just sit the fan on top. I had to duck tape off some of the back of the fan cowling.
Black duck tape would have been prettier but it’s also, inexplicably, a little more expensive at the moment. You could put the fan entirely inside, but then you have to do something to get at the switch.
We used an incense stick to generate smoke for a test (over the black mat by the front door for contrast). Lots of air was being pulled through the filter, leakage around the cowling of the fan was there, but minimal. At full power it matched the air speed through the filter of the box fan (put the filters at a 90° and the line between smoke going into each should be roughly 45°) so the filtration rate should be comparable. It’s still a little noisier than I would like, but thrifty at about ½ to ¼ the setup with 3 or 4 filters and a box fan, and takes up less space.
