My friend's house has HVAC whose air filter is a rectangular metal frame filled with washable fiber mesh. It doesn't look like it could catch fine particles.
We wanted to try an off-the-shelf allergen filter (e.g. Filtrete) but couldn't find a matching size at Home Depot or Lowes. The closest fit would leave a 1" gap (about the size of the frame) on each side. Is there an adapter to wrap around it? Is this a specialty size not sold in stores? Or do we need something else entirely?