How to heat my bettas water other than using a heater?

Hi, I have a single betta in a gallon tank. His water is at 72 degrees and Ive read that that is close to being too cold for my little guy. I wanted to get a heater but they only have them for 10+ gallon tanks. I don't want to cook him by using a heater that is too powerful so what are some other ways I can warm him up? Or will he be fine at the temp he's at now? He's not lethargic or any less energetic as I have read they get that way when theyre cold. He's just as spunky as when I brought him home 5 months ago, however, my other betta just died and he was in similar water conditions : ( I dont know what caused his death but I dont want Leroy Jenkins to suffer a similar fate!