Every now and then, at the field I get questions about these. They're a quick, easy, and cheap way to keep from losing those green ten round tube caps, and a LOT of people make similar things. This is just one way to do it.
You just need to drill a small hole in the lid, stick a zip-tie through with the locking head on the inside, lock it in place with a second zip-tie on the tail (as close to the end as possible), clip off the extra, and tape it to the tube.
Zombie Defense Force, Local 42 #61 "Darwin"
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I made a bunch like this, possibly based on instructions you or someone posted elsewhere. One thing you can do is take a razor blade and cut down the head portion of the first zip tie, so it's just a "L" shaped piece at the end. Basically cut most of the head off. That keeps it from pressing on the stack of balls, possibly deforming them.