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I made a wooden block the same size as the front of my intake duct (I don't
know what a duct to a 320/360 looks like). I made sure the angle of the
opening had the same relation to the sides of the block as the duct had. I
formed 1/4"hardware cloth around the block and soldered it. This wire cage
goes on the outside of the duct and extends about an inch back. I then
pulled some filter material from Boeing around the outside of the cage and
tie wrapped it in place and cut off the excess. the foam pushes against the
cowl opening, but allows independent movement. There is no way the filter
foam can be sucked through the hardware cloth cage. I change this every 50
hrs. I see no vibration wear on the cage and little on the fiberglass duct
from the cage.
I noticed the  outside filters of the Honeywell room air look like the same
as the stuff I got from Boeing, but have a sticky substance on their surface
and are cheap. One Honeywell outside filter would make enough filter
elements for about 4000 hrs of flying. I tried cleaning a used Honeywell
filter with a shop vacuum, but the sticky stuff kept the dust stuck to it.
The vacuum would not pick up the filter material, which means it has very
little resistance to air.
Bill N7WB