I'm considering modifying my ventilation system to draw fresh air from outside rather than the ceiling cavity. The motor unit has a 220mm diameter, so I guess I need 250mm conduit. I found a place that will sell it to me for $90 + GST for 6 meter lengths, though of course I need 7 meters. Plumbing world is $50 for 3 meters. Both seem pretty expensive. I could probably get away with 200mm conduit, with some liberal use of duct tape.
They also sell filter boxes for $325 + GST. It's a metal box with connectors for conduit (150 or 200 mm I think) and a simple cloth filter inside. Can anyone point me at something cheaper?