I really like Ronmar's computer type box fan(s) idea, but I think I'd go with the AC version for simplicity. Very low power, redundancy by design, and a good match for our very limited airflow needs. You could consider eliminating the thermostat switch, also. If you had 3 set close to the radiator, I don't think you'd even have to make a simple shroud, and any one failing would still leave you with plenty of airflow. They are available for 120 or 240VAC. mpja.com has had them cheap, surplus, in the past.
I woudn't want the 1500+ watt start surge of a 500 watt attic fan that happens just as my water well submersible pump tries to start, but then I've only got a 3KW ST generator head. The DC auto fan would be better as far as start surge issues, though again it is way more fan than we need.