Chopped fibers and resin makes a great filler, I use milled 1/32" fibers and mix them with epoxy to form a thick paste. If you find that you want a paste that can be used on more vertical surfaces, trixothropic silica (Cabosil) makes for a very dense paste.
If it should not stick permanently, use plasticine, modelling clay. Pelikan Yellow Wax Filler is a great product for that.