I know what you mean. Those openings are way too small to do a perfect job. I couldn't get seam sealer all the way to the point that it curves down on mine, but did the best I could. Mine hasn't leaked yet.
The way I got it all the way down the curve??? I put a huge glob of sealer on the end of a long flathead screwdriver and basically pulled it towards me along the seam. It wasn't pretty, but it worked great! lol