I had good luck using email otherwise you flag down the rangers you find. :)
I tried getting it there and they tell u to do it online. Most rangers have never checked mine and iv been going for years.
There is an entry gate with a person there and more than one Park Ranger there in a nearby office. It appears to be the main office from what I could see. Plus an adjacent food cafe' so someone there I'd say most of the time.
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