Jan's way is best and lightest. I glass the fuselage and then glue on balsa fillets. Normally I cut the fillet using whatever thickness of balsa is correct and use the root airfoil as a pattern. After the saddle area of the fillet is cut accurately I either trace the outline of the fillet on the wood or if it's a constant fillet (doesn't get larger or smaller from one end to the other) then I run it through my scroll saw with the blade at 45 degrees.

I glue that to the glassed fuselage with the wing in place. I use a micro-balloon/epoxy mix to make the wing saddle perfect. Now I sand the fillet to shape. After the fillet is shaped I glass it.

Sounds like a lot but it's not that difficult.