First time homeowner here. I'm having a heck of a time trying to find an air filter that fits my furnace. The previous owners had a filter in, but it turned out to be the wrong size (16x20x1). I measured the gap for the filter and it appears to be 1" thick, about 18.5" wide, and about 15.5" deep. In case this helps, the furnance is an Evcon model GF8100C16MU11A. Couldn't find anything on the net about this sucker. Does the filter have to be a snug fit, or can there be a little gap? And would it be a problem if the filter was too tall?