I was playing around with my light meter the last time I changed my bulbs, testing different screening materials. Egg crate blocks 15-25% of the light, depending on the angle of incidence of the light source (I tested using a point source, so to LockeOak's point, T5s would be on the 15% end of that range). A single layer of bird netting only blocks 5%.
+1 for the mesh top, but Anthonius is right - better egg crate than a carpet-surfing goby!