This will not be of much help. Dick. Two obvious problems in designing (or redesigning) your own mufflers:
1)- We are trying to simultaneously remove waste gasses from 300-400+ bhp engines thru sound attenuators (mufflers) the size of kid's lunchboxes due to lack of space, control the noise from combustion explosions very close by us, and also maximize horsepower from the engine. A dynomometer is a necessary tool for testing power gains or losses IMHO. Everything else is a pure guess.
2)- We need some way of quantifying any change in noise. No one including pro builders of mufflers ever mentions gains or losses in noise levels except subjectively. There are standards for exhaust noise testing with a dB meter but it will not separate 'drone' from other exhaust noises. Depends on your hearing acuity and personal noise tolerance. For instance, I can stand far more volume of Beethoven than what passes for 'music' these days.
Hot-rodders found eons ago that stuffing most any muffler with fiberglas strands or steel mesh works well but not for long. Heat (maybe 350F), vibration and exhaust pulsations soon tear them apart & blow the little fragments out the pipes. Back in the '50s we used brillo pads with the soap removed, or curly steel lathe shavings begged from machine shops . A few muffler mfgrs made take-apart mufflers back then so they could be easily 'reloaded'.