This might look like a joke product, but designer Paul Kweton has actually built and used the Ring-O-Star (ho ho) bike cup holder ... even get it with a "Rail Mount Adapter", which should clamp ...

I f you thought the handlebar-mounted cup holder was a bad idea, then you're going to hate the Beltclip, a $5 cup-holder which mounts on your belt. Yes, this plastic bracket will precariously hold ...