Check the airlines carry on requirements, it is not just about size but also weight. I recently flew from Thailand to Cambodia and the weight limit for carry on was 15lbs. With some of the better luggage today, they can weigh half that much empty. As mentioned already, I checked it in, put TSA approved locks on the case and all was fine for Angkor Wat.
Unfortunately because of the weight limitation I was unable to bring all of the gear I would have liked to.
That is odd they called it on a weight limit, who was it that put the restriction on it? I've not flown into Cambodia but I do Thailand at least once a year and never had an issue until last year when I went through Taiwan (first time going through Taiwan) and they said my 18 inch tripod was too long not too heavy. No big deal, I just check it and all was fine. I had plenty of time and the team at the airport was plenty helpful. I made sure is was checked on the return trip. So I guess the answer is to use check baggage and not worry about it. :-) Thanks for the tip.