What are the requirements for opening a bank account in Thailand for expats without a long-term lease?

I bet they want the lease to prove your residence—so the immigration certificate of residence would be fine. Just head to your nearest mall that has several banks. This will help in the future as sometimes you can only do certain things at your home branch. The banks in malls are open all the time versus the big banks in standalone buildings. In Bangkok you would need to go to immigration in Chaeng wathanna—u could just pay the agent (like Thai visa center) to get the certificate, which is what I did. You will need a tm30 from the owner of your Airbnb. The cost is just 4,000 approx. doing it yourself of course saves money but u also waste half a day and then another few hours later to pick up the cert. for me, paying is worth it since I’d rather be doing work online versus sitting in traffic or waiting at immigration. Good luck!