I ended up at a tailor across the street from the Marriott (used to be Riverside, now I think it is called Resort & Spa). The shop is called Alex Fashions. They are less expensive than tailors in town, although they mentioned depending on which location I was at (they had a few around bkk) the prices were higher at the airport or in city center.
I was limited on time that trip, so had the 1st fitting at the store, a 2nd fitting with the suit made, in the restroom at BKK while changing planes from Siem Reap to HKT, and finally picked up the suit when I came back through bkk.
Great service, especially to meet me at the airport.