I have made a few molds myself. As a suggestion, blacken the bondo with a candle flame held real close. It allows the lead to flow easier and will not allow it to stick to the mold. Make the second half of your mold from bondo also and clamp the two together using a spru hole from the side to pour through. Allow time between pouring to let your mold cool. You will get a lot more use out of the bondo.
A good source of material is tire weights - if you know someone in a tire store, they may save a bunch for you. I use a combination of pure new lead and junk from the weights. Just keep skimming the stuff off the top. It took awhile to come up with the proper ratio for weight.
I hope you are doing your melting outside and are wearing a good respirator mask. That stuff will do horrible things to the body and brain.
