@Michael *******
your embassy here either issues an affidavit of income from abroad notary letter (which the US, UK and Australian embassies STOPPED doing) and you use that letter
OR
you transfer in 65K baht a month, each month every month for the previous 12 months before you apply for an extension
There is no 'other proof' you can show so if your embassy doesn't issue the letter and you want to use monthly income you have to do international transfers