This is what I've done so far today. I need a break!

I "sculpted" and added the fillets at the back of the turtle deck at the verical stab. This shows one with body filler added to make smooth transitions between the parts. I used Bondo, which may be a little heavy, but very little will remain after it is sanded. I was worried that other types of filler might crumble in this application.

This was a little bit of a chore. To shape the fillets to the stab shape, I laid the fuse on its side on a towel, put 100 grit sandpaper on the stab and rubbed the fillets up and down on that so they would fit the stab. I then rough shaped the outside of the fillets with a razor plane and sandpaper. Then I attached them in place and sanded some more.

This picture looks pretty messy, but this is how the job gets done.