I did this last year. You can go to an agent and get a ticket to Siem Reap, Might have cost about 2k but forget. You will change bus a couple time but its all very easy to do. And its a vey nice town and of course do Angkor while there. Every time someone asks this everyone says just go to Immigration, but a side trip like this can be the highlight of your trip, was for me.
Thanks I actually was not aware they can ask to see 20k. Not a problem as for hotel I always book at least first few nights, also that is on the arrival card too.
Yes my other trip , ticket was actually 31 days and I got a lot of grief departing , Canada, from the airline who let it go after a lot of talk. But in BKK they didn't say a word . Thailand WANT us there overall.