Reason I was looking at the ones I linked to is because they are $8 for 10 filters as opposed to $6 for one. But I wanted to check to see if they were any good first which is sounds like they aren't. I have a filter panel on the front of my case that's a mesh fabric and does an ok job. I was thinking of adding a second filter to each front panel fan for more filtration as long as the air flow isn't too restricted. I was also thinking of adding another fan to the side panel to add more positive pressure to the case but there's no filter at all in that location so I need to add something there for sure if I do actually add a fan rather than just sealing off the vent.
Perhaps I'll try some HAVAC filter and hot glue and make a filter for the side and then figure out something for the front. Those filters are cheap enough and I might actually already have some that I can try and see how it works out.