I have a cheap old ventilation system that I've just changed to take air from outside instead of the ceiling cavity. My ceiling cavity is super dusty and has everything you can think of - rat droppings (apparently, very old), wool insulation, fiberglass insulation, cobwebs, treatment for bora, etc. If you're up there for more than 10 minutes every cavity fills with black stuff. The filter needed to be changed too often and it made the house smell a bit. So I changed the ducting to pull air in from the outside. I think it's 140mm ducting. There's no filter in line, just the plastic thing in the eaves that bugs can probably crawl into. I'll put some kind of mesh in there some time but i'll reduce airflow - I may get a booster pump as the system has a relatively small fan/motor.

I'd quite like to get a filter box for it, but when I asked a ventilation company it was going to be around $150 for a simple metal box, not including the filter. Can anyone suggest where to get a filter box and effective filter for a reasonable price? I don't mind a bit of DIY - buying a plastic bin from a hardware store is fine, but not sure I can get the duct to reliably stick to that, or how to fit a filter.

If there's any way to get a draft stopper in there, excellent. Otherwise I might have to look into the sock type units Niel talked about in this thread if I can find them - one for the ventilation system, one for the bathroom, one for the kitchen extractor.