Hey Bad Habits, could you not separate the external ac intake and out take by using a slim piece of wood or metal between the vents to segregate the air one releases and the other draws? If you see what I mean that way you do not need to do anything internally just externally? Or even just using some slim line box section just placed under one of the external metal ducts you already have and running along the outside of the shed to another location?
Peace
Peace