he might need a financial proof for the application to the 90 days Non-Imm-O visa, not only for the application to the extension. I can't open the website of the Thai Embassy in The Hague to check, though
applying for a 60-days Tourist Visa at the Thai Embassy in Vientiane, is the best option. You need an appointment, though, and the processing period can be up to 3-4 working days. NOTE: In Vietnam or Malaysia the requirements for an application to a 60-days Tourist Visa are much stricter and more complicated than in Vientiane
in case your Tourist Visa arrives in your email after you already entered Thailand visa-exempt and got stamped in for 30 days, you can extend one time for 1900.- THB for 30 more days. Before they are over, just do a border bounce and activate the 60 days stay permit of your Tourist Visa (keep in mind that a TR visa must be used for an entry within 3 months from its date of issue)
if you are in Hua Hin, you can do the border run with a "visarun agency" via Ranong and Myanmar, and back on the same day. This is the only land/sea border to Myanmar that is open for foreigners
Visit the "Thai Visa Advice" group, use the search word Savannakhet multi entry and the requirement for the financials will pop up. However I was wrong, there is no seasoning required and it does not necessarily have to be a Thai bank account. Only for the single entry Non-imm-O marriage visa, there is no financial requirement.