From what I've read, the thickness of the inner tube isn't important. It's been referred to as a tube and a rod. The only thing with the outer tube is going to be your gap. The ideal gap mentioned was 1mm. Depending on the outer diameter of your inner tube, using a thinner outer tube would increase the gap.
Then again I'm new and could be wrong, or just partially right. Maybe someone else has a better explaination.