Failing to find an affordable muffler to quiet down my big block truck, I made my own mufflers. Mine were considerably larger than the ones you are attempting at 12x24 inches. I used scrap aluminum .125 sheet from a work project. I did copy some of the internal structure of some other more popular mufflers and created a 3 chamber design with the requisite deflectors, tubes, and baffles. Overall the project was very successful and I was finally able to hold a conversation in the truck while it was running.
Good luck and let us know how it sounds.