If you haven't done so already definitely cover try to cover up most of the screen top with something aluminum foil works well for this use about 3 layers shinny side down, remember to leave some open space for ventilation, just remember heat rises so you will lose a lot of heat out a screen top.
a ceramic heat lamp, or nocturnal bulb such as red or black can also be used. I would recommend ether buying a fixture with a built in dimmer or add a rheostat which is the same thing that way you can adjust it for the correct temperature.

also what size tank do you have?