I thought I would explore something better and more efficient.
Not difficult, I sourced a 120mm brushless PC fan that was uniquely housed in an Aluminium frame for added weight. Price, £6 ($10) delivered, I added two steel grills front and back for safety and hooked it up directly to a 12v, 4 watt solar panel that I 'hung out' of the south faceing roof.
The sun shines, the cooling increases - not a massive volume of moved air, but welcomed and appreciable.
Current draw was around 280 mA, 3 to 4 watts of power.
I did connect a small 3 A/h battery also for full power when required, solar charged obviously ;)
As with any ball bearing based PC fan this is good for 7 years of continuous use - It does buzz a bit in very low light / power conditions, this might be damaging?? - advice appreciated as always.