I've just arrived at Bangkok from kuala lumpur doing a border bounce. I advise purchasing a dummy ticket showing you leaving Thailand again. I had one and showed it. Until then I was being grilled as to how long I was staying. Despite me telling them approx 1 month I thought I was going to be refused entry. The IO even called for assistance for advice. This was my first border bounce.
I flew Birmingham to Bangkok with Qatar. No questions asked about returning. I did however have a reservation from Bangkok to Kuala lumpar which I purchased for 10$US from an American site. Just as a safeguard