If you're building a 1X solar box, the radiators are worthless.
On the other hand, if you're building a reflective concentrator, the radiator is a cheap and superior collection point because it allows the heat to pass very quickly into the liquid (small parallel passages). Just make sure you don't let the fluid-flow come to a stop or things will be glowing in pretty short order.
Also, if you have it mounted in an insulated metal box with only the "hot" face side open to the air, you'll retain most of the heat you collect without it radiating away.
Old ovens have plenty of scavengable high-temp insulation. If you find a small enough radiator you could probably just take off the oven door and put the radiator inside.