Very good place is CO2 mixer made from inline water filter housing (see previous instructable http://www.instructables.com/id/CO2-mixer-for-planted-aquarium/), because is such a waste of volume (nearly 2l) used only for CO2 mixing(!).
Step 1: Bit of explanation
Finally I've decided to buy cheap thermostat with external sensor (cost about �11) and heater, replace original heater by normal glass one with thermostat built in, and place the heater inside CO2 mixer.
Why two thermostats?
The reason for that was security, and flexibility.
Lets imagine the housing contains about 1,5-2L of water and the heater inside is 100W.
Unless the water is flowing trough housing there's no problem. If you switch off filter (or something will stuck in pipe, might be thousand reasons) and there's no water flow, the external thermostat don't know about water temperature inside housing (because sensor is in the tank) and will heat until the water will boil inside. Housing, seals, heater etc might survive that accident but if I can I wanted to prevent stuff like that. For that reason there's two thermostats one (in heater) set up slightly more than normal temperature in fish tank (my is 26-27C, lets name it security thermostat), and the second one actual thermostat for aquarium temperature (external one).
Actually you don't need external thermostat but I wanted adjust temperature in fish tank.
With heater only inside CO2 mixer will be quite complicated (you have to remove water and unscrew housing, adjust temperature, and again, screw it back again, fill by water, deaerate whole pipes... just nightmare to adjust temperature for example 1C degree). So it's up to you. If you're sure about temperature the work will be less complicated than mine.