I had no problem getting the letter from immigration that you then take to the bank. Kasikorn bank was where I went and they told me about the letter I needed from immigration. I brought immigration a letter from my condo confirming my residence there, copies of passport, non tourist visa stamp, filled out the form request, paid a couple hundred baht, waited an hour for them to process, and left with a letter that brought back to Kasikorn and they opened me an account straight away.
It was easy once I found out what was actually needed, which no one had told me before.
I do think it’s easier in the areas with higher foreign residents. My Thai wife and I had tried out in the rural areas and kept getting the same run around that it couldn’t be done.
I was able to get it done by myself in Cha Am, along Thai drivers licenses once I had the right documents for immigration to give me letters.