I found just a handful of these online, and I decided to try my own design out with materials I already had. It's just a rough draft, so it's not really pretty, but you can get the idea. I think I might try my hand at making some prettier ones.
Materials: Elastic cord (could also use a hair elastic if it's the right size), Ribbon (or cotton belting or you can make your own strap with cloth which is a little more work) cut to about 2 feet, Velcro, thread
Measure elastic cord by tying snugly around a bottle or cup with the lid on
Put ribbon around cording and sew down with knot on the inside of ribbon (so it will look better and stay tied)
At the other end of your ribbon, sew on one side Velcro, then fold over and put other side of velcro in desired spot-- you will have something like a leash handle when they are attached
You can attach it to anything you can loop it through. My purpose was for when we are eating out or in the stroller to keep that cup off the floor.