If you can't get that natural airflow, that fan idea is a good back up.
So far for TD Towers, we have only the air flow through the roof void and it is working. However, I put the heat reflective insulation directly under the tiles AND had the insulation over the first floor ceiling joists (like one would do to keep heat in, in the west!) so that double whammy is probably helping.
It's a case of employing as many different methods as you can and hopefully the result will be a cool house interior.
As we all know, heat is the enemy. Another area it can be influenced is how you design in the windows. Massive picture windows to look out can be great but they will let in a lot of sunlight, heating up the interior. Also, where you place those windows in relation to the suns trajectory throughout the day will be a factor. Lastly, a decent roof overhang can cut out at least some of the associated sunlight ingress issues.
We went for 1m wide by 1.5m tall windows. single glazed, not double. All these windows open. for the days when you can make use of the air flow through. they are supplemented by working wooden shutters on the outside for security and light blocking as necessary. yes we are using them. The roof overhang is a generous 1.5m which does limit sunlight into the upper floor rooms. we are looking at the practicality of fitting something over the ground floor windows
With windows on all 4 sides on the house we cannot avoid that trajectory but... we have it rising facing the exterior of zone 1 and setting facing zone 3. this means the main living rooms - kitchen, dining and living rooms do not have intense sun when they are being used. Sun comes up on the side of bed 1 and 2 but by the time it could be an issue we are already up. those AAC blocks definitely help with heat insulation
The final heat influencer is...trees. What and where you plant them can shield the building walls from the suns rays. just make sure any root structure does not undermine your foundations. tree roots are pretty strong!
Hope this is useful.