I once made some. Started with a fair sized blocks of 6061T6 and ended up with 13 gram tongue mufflers. It was a lot of work, and I used a CNC milling machine. I also decided that I like tube mufflers better...mellower sound, less bark. Some engines don't seem to run well with tongue mufflers, on top of that. Volume is important, as is the amount of "leakage".

You should look for suitable mufflers from the usual stunt sources, such as Randy Smith, Eric Rule, Big Art, Brodak, Leonard Neumann, Jim Lee, RO-Jett, and some I've probably missed.  If you insist, you can make a (heavier) tongue muffler from DuBro exhaust extension stock. The large size fits K&B/Veco .61's nicely, and I seem to think there is a smaller size from DuBro that might fit some .40's.    Steve