microwave vent (rectangular to round)
 

I have no idea what I'm doing, so please bear with me.

I have an over-the-range microwave (it replaced a hood that used to be there), and I want to vent it outside. However, the microwave has a rectangular vent (10" x 3.25") while the existing vent to the outside is round (7").

Here is a picture:
http://s15.postimage.org/ol9ubx2o9/microwave.jpg

I have seen that there are some rectangular to round adapters that exist, but there is hardly any room in here to work with (the distance between the top of the microwave and the bottom of the round vent is only 6").

Any idea what I can do to make this work? Any links to specific parts would be greatly appreciated. Thanks!

may have to cut 7"round pipe higher to install 3 1/2 x 10" x 7" round adapter...ben sr