First, you should never cover any heating source with anything. If the reference is to the construction of the heater itself, you should never deviate from the manufacturer's design. The manufacturer has predetermined the best design.
It could vary from 150 to 500 depending on its size. If it does not bother you, you may find a used one which might cost you less and work well. Invest some time researching in different stores and the stores' credibility.