Wander through your local Home Depot (Merklin's favorite store), Lowes or DIY (depending on your continent) and look for door and window weatherstrip. Frost King makes a good line in the US, but any will do. If you have a black top, then get the black rubberized stuff - it won't leak. It comes in multiple widths and multiple thicknesses, so figure out the width of the channel and how much of a gap you have to fill (thickness) for the right stuff. It has adhesive on one side and can be installed in a couple of minutes with scissors.
Third best thing in the world after duct tape and silicon caulk.....