Go to your local computer repair shop, grab a bunch of free stuffed hard drives, pull them apart, and you've got some decent magnets.
The other day I picked up 100 neodymium magnets about 1/4" diameter, 1/4" long. They will fit down through the filter feed holes, past the ADBV, and catch stuff on the dirty side of the filter (I don't like the idea of magnets on the clean side)...100 magnets $31.