You can help filler to stay in place by making holes, sticking pieces of weld rod, or putting small screws in the area to be covered, but it's not really meant to be very thick. I'm sure you don't want to get involved w/melted lead, though.
The reason why i don't want to use bondo is because it cost alot of money for me. The brass i could try though. I have used silver solder in the past for filler and that work great.
Not sure where your from, and what it costs there... but around here a gallon of Dynalite goes for less than $15 and goes a long ways when your using it on bikes.