We've started using Cadence's umbrella stroller instead of the one her carseat hooks onto. It's great, but it doesn't have any cupholders or storage places. Instead I had to make something that would hang from the handles (without falling on Cadence's head like the diaper bag does when I try to hook it there). This is what I cam up with. It works great, and was super easy. I plan to make a couple more, one for my water bottle, etc. I also slid a clip onto the loop of ribbon for my cell phone and keys.
To make your own, cut a rectangle of fabric that will wrap around your cup or water bottle, going most of the way up the side so it will be properly balanced. Fold the top seam down a little wider than a piece of elastic and stitch. Fold the fabric in half right sides together and stitch along the bottom and sides, leaving the folded over part unstitched. Turn right side out and thread elastic through the folded part. Stitch the ends of the elastic together, at the same time stitching in a loop of ribbon so it is connected firmly to the elastic. That's it! I didn't mess with trying to close up the opening where I threaded the elastic through, and you can't even tell.