If you have not glued on the stab and fin, create a "dummy" stab and fin filler piece the same thickness as the surfaces. Cut these to the shape of the fuse, and tack in place with a couple of drops of glue. Then tack the filler blocks in place, and sand to correct shape, using the fuse as a guide. Then remove the parts & install the flying surfaces. Now the filler blocks will be the exact shape needed.
Hope that description is not too confusing. It's really simple, just sounds complicated.