Hard to say about the shroud, it might be a good idea, it might not be necessary. If you've already got more rad than fan, then there might not be much gain to be had. You could make up a ghetto shroud, just to determine effeectiveness. Just take a big piece of cardboard and cut like so:

Fold the sides down over the rad, and seal all the cracks with packing tape. It would at least give you an idea of whether you should consider going to the trouble of making a proper one with a piece of plastic or sheet metal.