Burning it in a pit would work but I'd give it a try with a sheet of steel corigated tin roofing. Maybe angle the tin enough for the lead to run off into a catch area. A stack of wood pallets on top of the tin then your sheets of plywood. Light it up with something flamable like charcoal lighter fluid, Don't think I'd stand in the smoke, vapors would be there. I've read about this being done long ago to recover lead bullets from range backstop dirt and wood.
woods