You should be able to get tons of lumps by taking apart a car/wreck and disassembling the plates/frames; though this requires quite a bit of welder charges.
Alternatively, driving an car into a big building will accomplish the same thing. If you can survive the crash.
Finally, you can assemble lumps using either scrap metal, metal chunks, or rebar. The first two you can get by smashing or disassembling most metal furniture, and the last you can get tons of by burning down a house. Note that this requires a lot of battery/charcoal for the recipe.