You can use just about any old can on the back of the muffler if you want. The cans that hair mousse come in are popular. Get bit of threaded and some nuts from the hardware store and screw it together. Use some RTV available at an autoparts store to make a good seal. Drill an outlet hole somewhere (location is not critical) and away you go. It's ugly, but it's cheap and works.

Of course, you can always just order a new muffler. Magnum parts are quick and easy to get, I should know, I've ordered enough of them.

http://magnum.globalhobby.com/
or
http://globalservices.globalhobby.com/part/21075001.asp

The second link is the parts list for the Mag .40XLS. It's 2 pages. Looks like a new muffler is around $25, a bit pricy. You could also try a Tower or other brand muffler. I don't know for sure in your case, but on the smaller engines, the Magnum muffler mounting dimentions match up with OS pretty closely. And I'm pretty sure the OS and Tower .40s have the same muffler bolt pattern.