They have now closed this office. You need to get the re-entry permit from the hall where you queue up to have your passport stamped. As this is after security check, you wonโt be able to get the permit if you are not flying the same day.
Get single (or multiple) re-entry stamp BEFORE leaving Thailand (it's before you do immigration) at Suvarnabhumi Airport. Bring along a photo of yours and THB (fee for re-entry stamp) - it takes no more than 30 min.