Probably the most efficient and quickest way of doing a muffler mod with a basic amout of tools, is to use one of those ration pack type can openers if to have one handy. In luie of that, borrow someones carving saw and simply "plunge cut", a well placed port.
If the plunged cut is not open enough, pry it open further with a crowbar, ice auger or large screwdriver using a discarded brakeshoe from a boxcar.
John