Not on the top. The reason for that is that cool air goes right under the radiator and up, being warmed by the radiator, then out through the front to be circulated in the room. A topless cabinet is less efficient. David Bedrosian of Watertown found out recently that it is hard to find 8-inch aluminum clapboards. He needed to find some to make repairs.
Trouble is, they are no longer made.
And they're open Saturday morning. The reflectors were popular during the energy crisis of the 1970s, when people were really concerned about heating and saving fuel, but aren't so popular now. So, you have to find a substitute. Just cut it to size and mount it on the wall behind the radiator. Or tape aluminum foil, shiny side showing, to the wall in back of the radiator.