Special attention to build the vents so they will be weather-tight when closed.
First, I ripped a 2x4 into 3 equal parts. Two thirds will make the vent window and the last third will be the stops inside the frame.
Here are the two-halves reassembled for added strength. Notice the overlapping in the corners:
Here is the vent with weatherstripping on the bottom with hinges installed. I wrapped both sides of the frame with 6 mil greenhouse film (4-year) and stapled then taped the edges. The entire assembly is less than 10 pounds and should work well with an automatic vent opener.