Shotgun slugs would work good, but they cost too much for a tightwad like me. I use a 20 oz claw hammer, with straight claws, Give it a good swing, claws first, then twist the handle 90 degrees. Makes a nice vent hole. I usually put 4-5 around . Trick is to swing the hammer hard enough the first time, or you just get a dent.

Edit-An air chisel("muffler gun") works good to get the top out. Much easier than a torch or Sawzall.