It really depends on the style you're going for. You wont need anything sown @ all. You can get away w/o sewing, don't let that hinder you.
The problem w/ cords is they can dig into the skin since they're of a smaller width and the pressure is not as spread out. If you can deal w/ that or think that wont be an issue, then go for it. I would use a flat piece of fabric instead. Make sure that it's something that wont lose it's tug overtime. As the same w/ a rubberband, if it's smaller and spread out overtime under long durations (time/tension), it will lose it's elasticity to some extent.
The biggest problem would be finding some type of anchor point. Waist stretching/extending is more difficult to configure, which is why
phallosan uses a ring around the penis.
I would just mimic that as that's exactly what you need. I don't see any other way to achieve this type of stretch other than making it tight around the lower abdomen almost acting like a stomach belt. Then attaching a cord from a point on that belt to the hook around your vacextender. That's a pain in the ass to do and might possibly be a pain in the stomach as well.