I use a wire frame holder I purchased off eBay for a couple of bucks that attaches to the handlebar. I was looking for something lightweight, small, and not intrusive. After installing and testing it on different size containers, I discovered the frame didn't have enough spring in it to hold some containers in place while tilting the bike in curves, so I placed an elastic band around the middle of the holder. Now works great.
I got this simple one at a local bicycle shop. It works well for stopping on my way to work and picking up coffee or a soft drink from a convenience store. It's too wide for a regular water bottle. A McDonalds large soft drink cup is about the biggest it will hold without it tipping over.
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