Visa extension letter for Canadians is not possible. I received an email from the consulate today.
"Issuing a letter to allow you to stay in the country is not one of the service we provide. We are unable to issue such letter because we can not prove whether your reason to stay is genuine. What we can provide is an affidavit letter where you need to apply for it in person at the Embassy in Bangkok. It’s a free of charge. You will need to bring your passport with you when you come to apply. The Embassy can not guarantee if the letter will be accepted. It is up to the Thai Immigration whether they will consider accepting the issued letter or not."
TLDR : Answer Summary
Canadians cannot obtain a visa extension letter from the consulate in Thailand, as per an email from the embassy stating that they do not provide such letters. Instead, they offer an affidavit letter that one must apply for in person at the embassy in Bangkok, though there's no guarantee of acceptance by Thai Immigration.
