I know there are many ways to do this but the method I've used for the past 15 years or so is SIG Epoxolite Putty. Get yourself a SIG catalog and check this stuff out. They usually devote a page on how to work with the stuff and how to make wing fillets, etc.

I also make small molded parts with it. One of my favorite tricks is to make ADF antennas. I cut out a profile from light ply, then form the shape with Epoxolite.