Not sure about them being open at that time. If I were in your situation I would go to the nearest immigration office. Multiple entry applications take almost no time at all. Even at Chiang Wattana
It was a little crowded today, but I got there at 13:00 for a residence certificate. Handed the paperwork in and was on my way at 13:30. Came back at 15:00 to collect the finished product
What is usually the big holdup is people not having the correct paperwork. I was guilty of this today. Had to go next door for a copy