I had the same question and emailed wiggle Q&A's but never got an answer. I use a clip from an old pump which sort of fits, and I make sure the velcro is tight.
The clip that came with mine had plastic ties that would enable it to be mounted next to the bottle cage, you could put them just above the bottle cage bolts to be sure it doesn't slip down.
I wanted to avoid cable ties so went for my kludge. I have seen much fitter and more pro looking riders with the same and similar pumps mounted under the top tube using cable ties though. I'm not swish enough to carry that off though

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