ORIGINAL: TLH101
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.
I've done this on plety of other planes. Unfortunately, the vertical stabilzer is glued in place just like the instructions call for. I could probably still manage to carve something but I'll try it the way it was designed first. Also, if the fillets were in place before installing the horizontal stab, it might be difficult to get the stab hold-downs in place.