Radiator (cover) will restrict radiation so..
As was suggested make a sheet metal box on the back and sides to prevent heat transfer to the wood. Remember that warm air rises so encourage that by leaving holes at the bottom or even a small fan like from a computer at the bottom will force the air up the back and over the top. You will be restricting the "radiation" area by covering the unit. Now I'm imagining an old steam radiator, whereas you may have a base board heater...i don't know, but the principles are the same. A photo would be helpful.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 11-25-2010 at 11:27 AM.