You’ll want inner tube to reach deep into muffler with perforations at far end with closed tube.
Perhaps easiest to get pre-made universal unit and modify to fit. Slice off mounting ring to result with flat cap that’ll fit right into end of muffler.
https://www.ebay.com/i/222190529373
As mentioned, inner tube should be about 20mm to give exhaust room to flow forward to end with perforations.