[dredmorbius]

The problem for areas presently affected by high PM2.5 concentrations, most notably Australia, is that materials which are commonplace in the US, box fans, but especially box-fan-sized square HEPA furnace filters, are rare as hen's teeth.

So alternate designs using available materials are required.

(Or someone might drop ship a few containers worth of box fans and HEPA furnace filters down under.)

[brnt]

Same problem here. I've just spent close to an hour trying to locate a box fan for sale in Europe, couldn't find any resembling those I can easily find on amazon.com!