Richer, if you don't put a fan or a louvered top, ect inthe casing you are going to have problems from the heat build up. the main problem will be a shortened life spam on the ballast its self. also if you have a capaciter in there fopr the ballast you will see it fail quite often.
the best way for a smaller enclosure like that is to get a computer fan grill on one side and on the other side mount the fan pulling air out of the box.
or you can do like me and leave the lid off all the time
Steve
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