Jacs,
I built the fillets the same as Zpat, all out of balsa. There are two 1/32" thick ply sheet bases that get glued to the wing contour forms located inside the fuse. These extend out both sides of the fuse. There are then 3 formers that get glued to the top of the ply sheet, on each side. The location is important and is shown on the plan. The wing should be attached to the fuse with a piece of wax paper between them when making the fillets. When gluing the formers in place, make sure they cause the 1/32" ply base to conform and lay flat against the wing. The plan should also show a 1/32" ply strip that gets glued to each side of the fuse, following the tops of the formers. Starting at the front, there are 3 balsa sheets that get glued against the ply strip at the top of the fillet, follows the contour of the formers down, and then gets glued to the end of the 1/32" ply sheet base. These 3 pieces butt against each other over the center of the formers. You will have to wet the outside of the balsa sheet to keep them from cracking as they follow the contour of the formers. Take your time on this to get a good fit. Some balsa filler and sanding will complete the fillets.
I will be gluing the ABS plastic pieces about 1/4" overlap on top of the front of the fillet. The covering will then butt up against the plastic.
Mike