3/4" cold chisel and hammer works great, and is pretty fast. Just remember to wear ear protection, it gets loud.
Use it like a can opener, slightly angle chisel so it's not punching through but cutting, and angle the chisel blade towards outside of drum to get as close a cut as possible.
Modded Brinkmann Gourmet. Basic UDS. Rescued 22.5" OTS. Weber SJS. Stubbs. B&B Oak Lump.
Cannondale 2009 F8 (for burning off all the great BBQ)