While you could, possibly, get another entry added to the existing visa, or purchase a new, double-entry visa, getting a re-entry permit should be relatively simple, straight-forward and stress-free. I confess I've never done this at the airport, I did it (multiple re-entry permit) a few weeks ago at Div. 1 Immigration at Chaeng Watthana, and it was simple, and took 30 minutes. FWIW, a re-entry permit takes up one-half of a page in my passport.