This will give you a good idea the difference choices of addatives you have for various resins.
http://www.shopmaninc.com/fillers.html
Bondo is cheap and not very strong. If you're going to use it . . . do what skools said . . . mix it with some Polyester resin.
If you want to mud that gap milled fibers, cabosil, and epoxy mix will give you the strongest goop. Make sure everything is dry and that you prep and clean the surfaces. Tape off the area to keep the mess contained and pull of the tape when the repair is still tacky.
Warner - it's a big no no to use polyester over epoxy . . . and you can't mix the two resins.