The only thin wall pipe of the right diameter I could scrounge up easily was fencing material. Its very low strength alloy. That's fine though because it easier to form bell mouths on the the baffle tubes.
Here the pipe is being rolled around breaker bar handle.
Current Stable : 950SM Turbo! GSXR1000 RT-P1100 VFR800 RXC620 motard EXC520 CR500AF CRF450R Plated! CR250
Past stable : 90 +