Wood filler can be used to make smooth fillets, but it won't add anything to the strength of the joint. Epoxy mixed with microballoons, as Boosterdude suggested, works very well, especially because the fillet can be sanded smooth afterward. If you are going to swipe an epoxy fillet smooth, though, put on a nitrile glove first; don't swipe it with an unprotected finger. But it is tricky to sand fillets for something as small as a launch lug, and mixing up a batch of epoxy just for the lug is a bit of work for just a little part. Air bubbles come from getting air trapped in the joint as you place and position the lug. Putting a thick bead of glue on the tube (as opposed to the side of the lug), and then pressing the lug down solidly, sliding it around very slightly to give it solid contact, and then swiping the sides of the joint clean with your finger immediately afterward can help lessen the chance that you will get air bubbles around the joint. Also, using a glue that has a smooth but thick consistency with a lot of solids, such as Titebond Molding & Trim Glue or Aleen's Thick Designer Tacky Glue (in the white plastic jar with the pink label) will also work well. (But be sure to swipe the fillets smooth immediately after application, because both glues set up fast.) Titebond M&T Glue is similar in appearance to, but is not the same as, its wood glues. Aleen's Thick Designer Tacky Glue is a type of white glue that is VERY thick in consistency. (Too thick to use in a squeeze-type bottle, in fact.) You can find it at a crafts store, such as Michael's, or in the sewing and crafts section at Walmart. Using these types of glue is similar to using the epoxy+microballoons mix, except that they can be used right out of the bottle or jar and don't require mixing beforehand.
MarkII