I understand the updated bank book situation apparently must be done on the day of the application, but what is the bank letter that they are requiring? 
I think the bank statement does not have to be on the same day, I think, like the gentleman said you can order it a week in advance and pick it up on the same day as your immigration appointment, but they do need to see your bank book transaction that was made on the same day of your application.
From what my friends who have encountered that problem have told me they had to show proof at immigrations that the money came from outside of Thailand. They would suggest to me that I transfer the money into the account from your home country account. This was at immigrations in Hua Hin