> Don't think you got the picture. filling
> the gap above a window AC on windows that
> slide left/right?

Oh. Turn the unit sideways, then expand
the wings.

Just kidding. Plywood is the way to go.
Buy plywood with the thickness of the window
frame. Paint the plywood. Buy fours strips of
aliminum strips the length of the plywood.
Fasten them to the plywood so they overlap
the sides. Put the plywood in place. Close
the windowns so they enter the channel you
have made for them on each side of the A/C.
Then use security sheet metal screws to
screw the aluminum strips to the window
frame. Caulk edges.