Have you got the fillets built yet?

When I did the Mustang, I wedged some pieces of triangular balsa between the fuse side and the fillet base, then used them to glue shaped pieces of 3/32 balsa to form the surface of the fillet. Then I thinned out some balsarite with a little water, troweled it into the corners and smooth things out with a spoon.

Another way to do it (at the expense of more wood and a little extra weight) is to glue a length of 1/32 plywood under the wing saddle as a base for the fillet, then countour it to the fore/aft shape desired. Then build up the entire fillet structure using pieces of 1/16 balsa soaked with some water/alcohol so they bend to the necessary shape. After the whole thing dries, go in with sandpaper wrapped around a dowel (or if you're really brave, a Dremel with a sanding drum!) and sand the concave shape into the fillet. Then you'd have a smooth wood surface for the Monocote.