Chad,
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.
Phil