Run a fine bead of clear silicone in the gap. Tape off both sides with blue painters masking tape first to keep the silicone from getting where you don't want it. Then spray a little glass cleaner on your finger and gently tool the bead so its smooth. Glass cleaner lets your finger slide over the silicone without sticking or pulling it out. Pull of the tape gently away from the bead. Let dry for at least 24 hours in a dust free area. Once dry, water won't get in. Don't use the cheapo silicone caulk from Home Depot, and try a test bead somewhere other than your bike first. Any excess cleans up real easy after its cured and dry. Mess with it while its wet and you getter a bigger mess.