Just got new cabinets and a new microwave over the oven. I have a problem with the venting. The new microwave uses a 11" by 4" rectangular opening in the back center of the cabinet. The exhaust vent to the outside is a standard circular opening to the outside with a flapper on it to prevent air from blowing in. This circular opening is offset to the left, and it's bottom is about 2 inches from the bottom of the calinet. My problem is that the circular opening on the exterior wall is so close to the rectangular opening bottom of the cabinet that I'm having a very hard time connecting the two. They are so close nothing seems to work. In a friend's house, I saw what appeared to be a metal box setting up against the back of their cabinet over the microwave. To me, it appears like their builder solved the problem by just making one big box and covering both holes with it. Would this work? Anybody have experienc with this? How do you make the box?