Once the plastic bottom piece has been removed, run a bead of silicon rubber on top of the line that you just drew. Notice here that I changed my mind after drawing the first line - I decided it was too high, so I re-did my support piece (now a brick face-down with a piece of wood on top, rather than bricks on edge). Make a fairly thick bead of silicon, leaving no gaps as you go.
Now replace the bottom plastic piece. Put it in from the top, pushing it down (until it rests on the brick and wood support) so that it goes through the silicon. It should scrape the silicon rubber bead off the line slightly, piling it up beneath the plastic bottom piece. When it dries, this bead will act as both sealant and support.