You may want to just buy black egg crate / light diffuser. If you add the cost of white egg crate and the paint its probably more than just buying it black. Make sure its plastic.
Something like this
http://www.eplastics.com/B-EGG-375X2X4?gclid=COHFkOarnb0CFSqXOgodqQ8AlA
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Something like this
http://www.eplastics.com/B-EGG-375X2X4?gclid=COHFkOarnb0CFSqXOgodqQ8AlA
Sent using Monster Aquaria Network App