I have a friend that opened Bangkok Bank accounts (both Savings and Fixed Term) only two weeks ago. He booked a hotel room for one month, and negotiated a deal for a longer term deal, to be paid after the initial month was up..
The hotel gave him a TM.30 and a letter that he would be staying there long term.. He used that to get the Residency Certificate, and that, along with his passport was sufficient.
Unfortunately the hotel owner reneged on the agreed price for the six months, but my friend found a much better deal.
You will just have to try different branches. Ones that deal with more foreigners are often more helpful.