I've read of using jar lids, rolls of tape, or rubber balls for forming smooth fillets, but I believe the ultimate tool is a spoon. Mix up a batch of Epoxilite (or better yet, K and B "Microballons" mixed with slow ure expoxy to a dough-like consistency) and pack it in the area between the fuselage and wing (remember, protect your wing with Saran Wrap or use 1/32-inch ply wing saddles).

Then slowly draw the bowl of a spoon across the fillet, forcing the filler ahead of the spoon as you go. The final result needs little or no sanding, and the spoon conforms to the general curvature of the fillet very well. If the filler sticks to the spoon, moisten it slightly with hobby-poxy thinner or alcohol... it works great!