I like this idea too.
You could interrupt the pocket for the bottom rod for an inch or so, stitch Velcro onto a 1" strap, put an adjustable plastic thingy to snug up the length and a plastic hook for the end by the belly band. The strap would be removable when it wasn't needed. Or, you could just buy a spool of bungee cord material and tie it to the rod and the plastic hook. It would degrade over the years but would be easily replaced.
Anyone have a web url for the track it would slide into? Knowing the channel width would be important in determining the rod and fabric or vinyl thickness to use.
I suppose UV resistant thread would be a nice touch also. I don't know about sewing vinyl on a home machine. Shouldn't be too tough if you don't use too thick of vinyl or fabric.
West Marine sells clear and tinted vinyl with UV inhibitors:
West Marine VinylUV thread. I don't know anything about thread sizes, but this is probably pretty heavy for a home machine.
West Marine UV ThreadI wonder if an adhesive wouldn't do the job?
Vinyl Adhesive with UV resistance