What are the requirements for opening a bank account for a visa extension based on marriage to a Thai national?

Make it simple, your name only. Ask if the bank can designate your wife as the beneficiary of the account if you die.

If not, get a will, which is probably a good idea anyway. Things like designating her to decide your medical care if you are disabled and unable to decide yourself.

As a rule, you can use a joint account that includes your name. But when you use it, you need to put double the amount of money into the account. (only the average by the number of names is considered as yours)

In your case, you need to put 800K thb instead of 400K thb in the joint account to meet the financial requirement for a 1-year extension based on being a spouse to a Thai national.

But you'd better contact your local immigration office to get confirmation they accept a joint account. Some offices just don't prefer those.