Go to F&M Enterprizes and look at their 2 part epoxy filler (That Blue Stuff) It is very light and works great fo fillets.
I lay a piece of tape down along the top edge and bottom edge of the fillet. Then mix up a very small batch of this filler and lay it in with my finger to smooth it out. Then take some rubbing alcahol and go over fillet to get a smooth surface and a even fillet. Then when happy with the fillet pull up the tape and you have a very stright edge and let it dry over night

http://www.stits.com/store/crew.html It is called Super fill.
Steve