As promised, the setup, and two basic shots, just to show it does work.
The light source=LED headlamp wrapped around an empty box (who needs ring flashes and other stuff?). The camera=K-x. The Sigma zoom extension tube. The Vivitar 70-150mm push pull zoom.
I don't need a rail anymore, with both zooms it is very easy to acquire and maintain focus.
Shots at between f11+f16 (yes, I removed the aperture ball and aperture lever from the Vivitar)
Tube at 210mm Lens at +- 120mm (+-300mm/12inches from subject)
Tube at 210mm Lens at 70mm (+-60mm/2 and a half inches from subject)
Watch out bugs, I'm coming to get you!