Hi,
I'm sure you could use the sonotube as the upper cage. However, I think you would need to reinforce it with wood or aluminium rings so it was strong enough to hold the clamps without warping. If you have to add these reinforcing rings then the sonotube is not doing much more than providing a stiff (and heavy) baffle.
I like Albert Highe's 3 tube designs. In case you are not familiar with them here's a link:
http://pw2.netcom.com/~ahighe/17_5ultra.html
He describes his clamps about half way down the page.
BTW your two tube design looks great!
Rod.