I did the same making an appointment online. Note that appointments can drop in at anytime. I ended up booking mine at 5 am. When I went there were some people standing in line for walk in appointments. It's unclear how successful they were but I think some of them got in. Someone else would have to cancel or not show for that to work.
You have to go online to the website at either 12am or 5am to make and appointment, that's how I did it.
Thanks! Your answer is awaiting moderation.