Actually, those are straight-through mufflers are quite loud. I got meatballed at Laguna Seca even with dB-killer noise-insert installed. I've removed el-cheapo muffler from my street-bike and put factory muffler back on.
I like his idea of re-using OEM muffler. Just matter of finding one and grafting it on. Maybe Ninja 300 muffler?
Here's an example of really good OEM muffler from Honda CBR250R:
It has 3 separate expansion-chambers. When exhaust expands rapidly from tubing end, that removes lots of "bang" energy from it. Larger volumes will allow for more expansion and more noise-reduction. Along with 2.5 reversals and most of noise is removed without causing too much restriction.