I use the Marina Betta Mini Heater I got at petsmart for Kaoru. While a small covered tank will over heat I discovered by adjusting how open the lid is I can adjust the tempriture so I have no issue. It also remains pretty steady. It's not flat so a betta may nap on it but it doesn't get to warm so you don't have to worry about the betta cooking itself. If you go to amazon you can find many other types, but honestly I never liked the flat ones due to I never got one to work.
It worked for her when she was in a tupperware dish, a half gal, and she's in a gal. now.