Rocketpoxy is awesome! I had only made fillets with Titebond M&T or with Bob Smith epoxy before finding Rocketpoxy; there really is no comparison. It's a little pricey, but it is great to work with. It is really thick, so you have to mix it and then spread it out on piece of scrap cardboard or a paper plate. I let it sit for 20-30 min to get rid of bubbles, then I gob it onto the fillet area and, after settling for a few minutes, I shape the fillet with a plastic spoon. (Lots of people use a small piece of PVC, but I think a spoon works way better.) It should make a nice smooth fillet with little or no sanding required.
Thanks for getting back to me about fillets. I completed my last set on the Executioner last night. I use plain old 30 minute epoxy. I mask off the body and fin then make a tape damm at the top and end of the fin. Using a spoon is the same method a friend of mines from MIT uses so you are in good company. Tonight I will do fillets for the launch lug and then the Executioner is flight worthy. With the weather we have had I don't know if I will even have the chance to prime it. Just over two weeks before the Berwick, Maine April 15, 2017 launch.