A radiator cover is essentially just a box. Create openings to radiate the heat out of the cover. If you are worried about losing heat because of the cover, slip a sheet of metal behind the radiator to reflect heat out into the room.
Earlier this month I moved into my first radiator-heated apartment. The year old apartment has some interesting qualities… including old paint chipped radiators. Bumps in the road like these less-than-perfect radiators are certainly a design challenge.