For a small tank, everything sticks out like a sore thumb. Instead of using even a small tube heater, try the Aqueon Flat Heater that easily can be hidden out of the way, while providing heat to the system.
The Aqueon Aquatic Flat Heaters are available in 7.5W and 15W sizes that can handle raising the tank up to 6 degrees Fahrenheit above ambient room temperature. They are completely submersible to make them easy and flexible to use.
The 7.5W version retails for under $10 and the larger 15W version retails for around $15.