Why don't you find the right size freeze plug and drill some holes in it? Start with on large hole in the center. Then drill smaller holes around the circumferance until you have the sound.
We did this back in the day with our cars running straight pipes.
Basically that is what Kevin is doing with his db killer. Its a tube of perforated aluminum with a cap (probably a freeze plug) on the end.
Kind of like this.
https://www.revzilla.com/motorcycle...20H1LA70libwkJ5DQaAhqYEALw_wcB&sku_id=1272961
or this...