Very good place is CO2 mixer made from inline water filter housing (see previous instructable https://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.
Step 2: Materials and Tools
1. inline water filter housing (CO2 mixer) see my previous instructable https://www.instructables.com/id/CO2-mixer-for-planted-aquarium/
2. external thermostat with external sensor and heater (mine was with water level sensor so I had to slightly change it)
3. glass heater with built in thermostat (in fact these days is hard to find ordinary one without thermostat)
4. silicone sealant
5. Drill with bits
scissors, screwdrivers, etc...
In my setup I've replaced mains plug by one with switch built in. I recommend switch off heater before you'll start digging inside aquarium. Don't cut the corners, it might be dangerous.
Step 3: Asembling Part1
then Set up temperature at 26-27C and put some sealant on thermostat knob and around seal (because when the filter is working on pipe outlet is slight pressure)
Hint: Even if you'll setup the heater to 27C, temperature inside tank will be 1-2 degree higher
Then I've drilled hole in the lid of inline water filter (CO2 mixer ;-)), put the heater cable trough the hole and filled hole by silicone sealant from both sides (that was the trickiest part of the job).
Now was time to secure position of the heater. I've used cable ties and some plastic washers to keep the distance from outlet pipe (see Warning!).
On the other side of outlet pipe I've stuck in thermometer the same way, just in case to control the temperature inside mixer (but this is optional really. I had already unwanted thermometer lying around, which has been replaced by electronic one).
Then I screwed in the bottom transparent part of water filter, and check heater position (see Warning!).
The original heater has been cut off (or you can solder off from PCB inside Thermostat) from external thermostat, and I've connected cable from new heater in place.
Warning: Heater (in fact only heating part see picture) must be completely isolated by water from any plastic elements inside housing, otherwise will melt down the plastic and even worse finally might crack itself, water + electricity... not good combination.
Step 4: Testing And
I placed the sensor inside tank (hidden behind filter inlet pipe, with electronic thermometer sensor to measure accurate temperature)
Filled the housing by water, run filter and de-aerate pipes.
Quick check for any leaks. Everything's ok!
Mains has been connected and heater switched on.
Step 5: Summary
If you'll have good combination of background and pipes colour (black background, black pipes for example), the pipes will be almost not noticeable.
Next time drop checker, for checking CO2 level in the tank.