When I bought my home, my furnace had a 16x20x1 nominal-size filter installed, and that's what I've been buying for replacements. This size doesn't fit the opening very well; there are large gaps around the sides. I measured the opening at 16"x20"x1-1/4".
I assume that the gaps are a problem, as they will allow unfiltered air into the furnace. What can I do to eliminate them?