I just like a bottle of water or Gatorade so I use a mountian bike cup holder, It holds a 16-20 oz water or Gatorade bottle securely. It doesn't take up much space and I have been using them since my Valk days.
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Just got the same bolt that holds the brake reservoir in a little longer length and found a spacer that makes the holder stand straight up instead of leaning forward. Have used them for many years through many states.