I have something sitting in my garage that should fit bit it might be a bit beat up. its what my exhaust came in
why don't you hit up a appliance store, best buy, sears, menards, home depot, something along those lines. thats what i did when we moved to get larger boxes. and they are usually free
I've got 24" x 48" x 14" boxes out in Chaska. Or if you stopped by a muffler shop, they might have them laying around.