Brilliantly thought out mod Mark!
They say that "Imitation is the sincerest form of flattery" so today we've flattered you as best we can by using your clever design on my brothers "new" 114mm reflector.
So the 19mm polytube didn't look too oversized sitting next to the 114mm's much narrower mount rings we downsized the polytube to the next size down, ie 13mm.
This was a tight squeeze, in fact you can't get the screw end of the hose clamps fully inside this size tube, but we used 2 separate clamps like you did but 2 pieces of polytube, one slid over the strap part of each clamp and the joins butt up against each other at the screw ends.
This worked a treat! Best thing is Bunnings had 25 metre rolls of the 13 mm on sale for $3.98! (50 metre roll @ $13.98, so this was a real cost saver.)
Good on ya Mark!