I keep that exact same heater pictured there in my Kritter Keeper, and I have had no problems thus far. I think it's like Littlebittyfish said, as long as the heater isn't touching the sides, its fine.
I wouldn't get preset Tetra ones. I know a lot of people use them on here, but you can't adjust them if you need to. I don't think they're worth buying unless they're adjustable, unless you have no other options.
This was my first betta, Aphrodite (RIP), in her 5 gallon. I haven't got a full shot of the tank. :( I'll take some photos of my current tanks (Aphrodite's tank is now home to a male, Apollo). :)