I have this small heater (from Meredith's link) in a small critter keeper, with some live plants. The plants are in here until the tank is ready.
This critter keeper is not quite a gallon. The heater is fairly stable, but it does run hot -- about 85 degrees. You'd have to try it in a 2 gallon bowl, and check the temperature before putting the betta in. I tried to use the suction cups to stick the heater to the bottom of the critter keeper, but it wouldn't stick. Maybe because it's acrylic? Also, do listen to the directions and don't put the bowl under or even near a light -- when I did this, the water temp went up to over 90 degrees.
I agree with QTOFFER about the 29 gallon tank -- the betta would like it in there, but I have seen some tetras gang up on and nip a betta, so watch closely and move the betta if necessary.