You can fill it with a slurry of epoxy and micro balloons.
Then you can lay a 2oz Fiberglass strip across the gap/balloons.
Then you can sand the fiberglass strip and blend the joint into the cabin top and the tailcone.
The same technique can be used for the transition between the cabin top and the fuselage sides. A lot of folks use this to hide the blind rivets that hold the cabin top on.
That's the exact reason for the light weight glass covering. I wouldn't depend only on microballoons either for fear that they would chip out. The thin glass layer keeps that from happening.
I am getting ready to cover these transitions and was going to use flox........which I have been using for all of the fill areas where strength is needed...............is there a big difference between the strength when I use epoxy & flox vs. using epoxy and balloons.........and assuming I use the same glass?