An inside curve is definitely tricky, but that's a small one, so it shouldn't be too difficult.

I would start by adding a 3/8" strip about 2" long to the front of the curve (Like you did on the seams). Then cover the fin as you normally would, but leave some extra in that curved area for pulling. Heat it as you pull it just as you would around any curve. If you need to, you can cut a slit at a 45 degree angle, but be careful that the split doesn't travel too much as you stretch