If that's all the kit came with, you can add a washer and another silicone o-ring to close up the gap and get a tight leak-free fit. I had to do that w/ a thermometer or 2, So it would go like this:
Valve > washer > o-ring > bulkhead > o-ring > nut
Can you post a link to the one you have or some pics? Perhaps you got the cooler fitting w/ a longer nipple.
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