Reentry Permit - Chiang Mai Airport.
Exiting via Chiang Mai airport today, we went through the security check at 7:45am and located on the left side alongside the security point is the desk for reentry stamps. This is before you get to the immigration desk. We had our paperwork ready but it still took a good 15 minutes to complete. We were the first in line this morning but by the time we had our stamps there were at least 5 people behind (or to put it another way, an hour+ wait)
If getting a reentry from the airport it is a lot quicker than at Promenada but get there early!