How to design a radiator cover for good heat distribution?
A radiator works by heating a room with radiation and convection.  Radiation is smaller in percentage as opposed to convection.   The heat of the radiator is radiated into the room.

Convection works by sending the warm air above the radiator and drawing in cold below.  This circulation is the main method of warming a room.  Vertical airflow is required to maximize heating.

To that end to putting a cover on a radiator will effect both radition and convection.

To maximize radiation:


Add a metal plate behind the radiator on the wall to direct heat away from the wall.
For the front and side covers, they should have enough openings on the sides and front to radiate heat to the center of the room.  You want to hide your ugly radiator, but continue to promote good radiation. Decorative screens/grills work best.


To maximize convection:


Do not place any large objects, curtains, or any other type of obstructions that could effect airflow in front of or next to the radiator.
Allow for proper airflow at the bottom of the radiator.  The cover should have opening(s) at the bottom to draw colder air through the radiator.
Allow for proper airflow at the top fo the radiator.  The cover should have opening(s) at the top to promote warm air rising.


Some suggestions for the top:


Make the top a removeable cover
Make the top a cover that can open and close (much like a cabinet door)
Add vents/grill/screens to the top


If you add a top that doesn't have any openings, then some effiency is lost for the style and look of your cover.  Basically we don't want to block the airflow but we don't want to look at an ugly radiator either.