There are plenty of ways you could do it. I am about to post a report of a trip I did yesterday from Sisaket to the border Post at Chong Sa Ngam and on to Anlong Veng. That's as far as I went but there was a group continuing on by minibus to Siem Reap which is about 260km away. I understand that Chong Sa Ngam is the closest Thai border crossing to Siem Reap. Further West Chong Chom is more civilised and probably near the same distance.
I'll post as soon as I can sort my photos out.