Hi,
What I did for mine was cut a strip off of a SS sheet 1/2" wide and ran it through a body shop roller, it is amazing how quickly it curls up! 1/2" wide based on Dan's recommendations (10' Otherpower) i.e. a 1/4" for the magnet rotor thickness + another 1/4" to go half way up the magnets. This leaves the magnet stick out by a 1/4" to maybe help with cooling.
I am not the best SS welder in town....so after a few attempts at butt welding I decided to overlap the SS band by 3/8" making the welding much easier. I positioned it exactly 180deg apart on the magnet rotors to balance thing out.
I would not be overly concerned by the thickness. Mine has been working without a problem now since November 1st. Oh by the way thanks to the Dans, their new book is great!!
Hope it helps.
sly