I don't know whether Bondo has branched out from polyester-based fillers to epoxy-based, but you do want epoxy and you don't want polyester. Regular boat epoxy mixed with phenolic microballoons is probably the best bet, mixed thick enough that it stands alone without sagging (it takes a lot higher percentage of balloons than of resin - like 5:1 or better by volume). You will get a better bond if you mix the resin and hardner, stir well, take a small brush and pre-coat the area with a thin, unfilled coat of it and then mix the filler into the rest in the cup. When your filler is ready, fill the pre-coated hole. Since it takes so much filler in the mix to get it thick enough to keep it's shape, it doesn't bond or seal quite as well without the pre-coat of unthickened epoxy.