Jim - your muffler looks great. I wanted to do something like that for the OK but here's the problem.
The OK muffler is made from sheet metal. It attaches to the engine with the same 6-32 screws that hold the cylinder onto the crankcase. I did not want to add any additional mass as I was concerned that it might overstress the hold-down system.
Secondly, since the OK tends to run hot (no head fins), I wanted to keep the restriction to a minimum. The tubular muffler that I fitted has a relatively large volume as well as a large discharge hole. I wanted to allow the exhaust gases to expand as much as possible, thus dissipating energy while at the same time, minimizing back pressure.
In the future, I plan to add head fins.
Bob Z.