Great question, I'd like to know how others have gone with this. I walked past an outdoor court off Ratchada behind the Emerald Hotel a few times, where I've seen a few pretty lively games in progress. It's close enough to Huai Khwang MRT Station, wouldn't take longer than 5 mins to walk. I'm trying to upload a google maps screenshot at the moment but am having a few issues. Gimme a minute or two and I should have it sorted.