How to quiet down my 3" exhaust?

I once quieted a 340 enough to make the cops happy, by sliding a 2ft rolled up sheet of corrugated cardboard into the ends of the straight-section tailpipes, and anchored them with nails dropped thru the tops of the pipes. It passed.
I drove around the block and pulled the nails out, hit the gas and fired that cardboard right into last Thursday,lol.
I was maybe 22 years old.