At Udon Airport, there is a public transportation , van service at 200 baht per head. Just tell them you will go to Nongkhai. A lot of van service in different destination. Tell them Nongkhai. Inform the driver as well tjat you will go to Nongkhai Immigration. The van will stop in front of the Immigration Division. So convenient.
I flew via Nok Air on the same day of my appointment. I catched the first flight at 6am. I arrived the embassy 5 minutes before 9am. My number was 70. 6am is the earliest ( first flight ) to Udon. Nok Air or Air Asia.