Professional grade Expandable and Contactable tubing - similar to Slinky Pop Tubing
I use extendable rain-gutter downspout extension tubes during the rainy season. Very similar to your "pop-tube" but 3"-4" diameter heavy vinyl (and made to "send water through". They support their own weight reasonably well, but would sag if fully extended. If you only expanded some of the folds (like every 3rd fold?) it would increase rigidity.