What about using these? They have a perforated core instead louvers.
MagnaFlow 18129 - MagnaFlow Glasspack Mufflers - Overview - SummitRacing.com
MagnaFlow glasspack mufflers feature a free-flowing, straight-through, perforated core and non-fiberglass acoustic packing material. They will deliver a smooth, deep tone to your rat rod, street/strip machine, or classic musclecar. They are proudly made in the USA from 100 percent aluminized steel with precision robotic CNC welding.