Must I plea in open court? What is considered open court in Montana? ...In a justice court (not of record)?

I'm pro se. I plead guilty to an offense of not displaying a decal on my ATV. I was going to take it to a jury but decided to plead guilty in the judges chambers. The judge accepted my plea and pronounced the sentence in chambers. While researching Montana law, it looks as if pleas and sentences must be in "open court". It seems that the basis for this requirement is to uphold public confidence in the court so the public can see its workings. I want to now ask the the court to withdraw my plea based upon it wasn't given in open court. Should I sniff elsewhere or pursue it? Not sure how to argue the point.