I have used 3Ms 4000 UV caulk on mine there. It's like 5200 except UV resistant for where sunlight can get to it, and it's fast cure. However, I really wonder if I should make or leave some open holes for water to get out since the toerail is canted out there so water will run off if there's no panel, but with a caulked panel the water can collect there (until I plane it and it all runs back.
Doug