I went through this process a bit earlier than everyone on here as I went in with my embassy letter on 11 August and needed to return on 8 Sept (same day as Brian). Counter K at the building next to the tent. My embassy letter was also colour printed. So with no hassles I got extension stamp of 26 October. For those travelling there, you can take a 166 numbered bus from Victory Monument all the way to Impact Stadium and then walk 1-2 blocks the rest of the way past a few shops. There’s a few places to eat and have refreshments at Impact including a 7-11. The 166 bus does take an expressway route, but also goes past Central Plaza Chaeng Watthana first before looping back to Muang Thani