I've had them fit perfect, had some that I trimmed one end, others that I used cut to fit fibered hogs hair filters.
Some have a piece in between them that turns to cover filters at edges and hold in place.
Some I have to run wire in X shape to hold.
Each one is different and without me being there, cannot be more specific.
Your furnace calls for 16X20X1, but if it won't fit, then you have to adapt, just be sure no air gets by unfilterd.
Some people are Humbly Grateful, while some are Grumbly Hateful.................... Which one are you ?