My attempts at reworking steel mufflers have been less than satisfactory- little if any weight reduction to get a gutted container.
So I'm using some of these extruded aluminum enclosures.

An aluminum alternative DIY that needs some steel brake lining for compression tubes

http://www.mouser.com/catalog/624/1386.pdf
spec sheet: http://www.budind.com/pdf/hb4500.pdf
Part # Dimensions:
563-EX-4500 EX-4500 1.57 3.14 3.81 1.14 2.47 3.00
563-EX-4501 EX-4501 1.57 3.14 5.38 1.14 2.47 4.56