Hi Doc, I make little radiators, about that size for my hot air engines, mine are a bit scruffy, but they work.
I wonder if with those side plates, could you fold them up with a larger radius, or are you using channel as bought. With the larger radius you would only need to knock off the corners of the fins.
Don't paint the fins, the paint will reduce the efficiency quite a bit.
The radiator on this motor has a fan (12volt,ex computer).
Ian S C