Reader Bruce sent along this simple and useful hack to carry tools while riding. And if your intent was actually to play tennis and you were carrying tennis balls instead of tools, see this previous entry for a hack to carry your racket.
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I saw a recent Bikehack water bottle tool holder. For years I have been using old tennis ball containers to do that job. I recycle plastic (not tin) tennis ball containers with plastic lids that you get when you buy tennis balls. These tennis ball tubes of various brands often come in 3 or 4 ball size. They fit great in a bicycle's bottle holder, are easy to get to when repairs are needed, and are water tight. So if you have two water bottle cages and only want one water bottle then you can use the other cage to hold the tennis tube.
Some bikes will fit only a 3 ball tube due to frame size. On bikes that fit a 4 bottle holder you can probably fit two tubes in it depending on tyre size. For the 4 ball tubes I usually put a small velcro strap around the top of the container and the bike frame to stop it moving around sideways - depends on how tight your cage is.
In mine I carry a spare tube, levers, patches, multi-tool, disposable gloves, band-aids, zip ties, elec tape and about 1m of string for tying the bike seat up to a tree branch or fence when repairing a back wheel puncture.
I find these to be better than bike shop tool bottles. The wide opening is easier to get things in and out of than using an old water bottle. I carefully remove the metal airtight seal before use. Be careful that there is no sharp edge left inside the opening when you tear off the metal seal - pliers or the edge of a screw driver can fix that if necessary.
For my night time commutes I line the inside of a clear tennis ball tube with a sheet of reflective material. If you ride in very wet conditions you might run some tape around the top of the bottle overlapping the lid, normally I don’t need that.