I have a lennox cbx27uh-036 air handler and a 13hpx-030-230 heat pump. I have to install it mayself because I can't afford someone else to do it. however I thought there would be some kind of direction in the box and there isn't. How big do I need to make the plenum and can it be made out of duct board since that is what I am running. Also how long should I make the trunk line stay as big as the plenum before I can do a reduction?