In looking at the physical dims. of my pending system, I find it surprising that my furnace (20" x 20" outlet) with my matched coil (20" x 20" inlet) actually only has a 14" x 16" inlet due to the drain pan. That it a huge restriction. And, that drain pan is essentially a flat surface. Why don't the manufacturers at least make a kit that allows the installer to add some sheet metal to the inside of the furnace to "funnel" the air to this smaller opening, making it more aerodynamic? It is a 400 square inch to 224 square inch "hard" transition. Could at least make it more gentle with some funneling. reminds me of a 40 year old semi truck with a flat front end. Do any of you ever fabricate your own "funneling system"?