I used .010" thick stainless for the inner pipe. The toughest part was bending up the 3/8" wide corrugated strips between the inner and outer pipes. The other problem is the welding; fortunately, I had access to a resistive spot welder at work or it wouldn't have been possible to make one economically. I've made outer pipes from .010" thick stainless and aluminum. The aluminum is easy as you can tape the seam with metallic tape.