victa, the muffler needs at least one internal baffle even to be vaguely acceptable, let alone quiet. A baffle is a plate transverse to the body of the muffler, with a hole in it out of line with the inlet and outlet ports of the muffler. This breaks up the shock waves inside the muffler. You can make a quieter muffler by having two or more baffles, with the holes not lining up. (A gun silencer typically has ten or more baffles, but has the disadvantage of having to have the holes in line, so the bullet can go through them).
A well-designed 2-baffle muffler in a steel pipe can be fairly quiet, but is likely to trap acids and rust quickly. It can also put excessive loads on the attaching screws due to engine vibration: if it is cantilevered from the port rather than having support brackets, it needs to be very, very light.
Why not just find an old muffler that is suitable with minimum modification?
