Just for S's and G's one time, I adapted a stock OS muffler to a ST. 51,( the newer G-51,) which has the same bolt spacing. One hole of the OS muffler was stripped out, so it became the guinea pig. I took a Dremel tool and cleaned up the back side of mounting flange of the muffler, so I could put a nut of some kind back there. I think I used a blind nut turned around backwards, and had to file one side of the nut flange flat to fit up right next to the exhaust stack. Then I used the Dremel cut off wheel to make a slot  on each side. I used 4-40 bolts long enough for the job, attached the muffler to the engine, and centered everything up and tightened the bolts. It fit up pretty nice, so I mixed up some J-B Weld and coated the blind nuts to hold them in place. I haven't tried it out on a model yet, but the reason for the effort was just what you are looking for if needed. I like the volume of that kind of muffler so the engine can breath better and it should work well.
   Another option  for you would be to find an older Big Art tube muffler, I think. Art had the tubes made by extrusion process, I believe, and has some cooling fins on it. They are not very heavy, but probably weigh more than a tongue muffler.
   One more option would be to get some of that Du-Bro muffler extension stock, that is the same as that manufactured muffler that you have, and cut it to the length you want. Drill and tap mounting holes on one end, and make a cap that bolts on the other end. Drill exhaust exit holes as desired, and/or the end cap can be shimmed out with flat washers for the exhaust exit. Lots of options with this method.
    Good Luck and Type at you later,
     Dan McEntee