If you feel the plastic in the holes, you can feel one edge is wider than the other. The wide side goes up (facing the light).
I read a while back that eggcrate in the tank can cause algae problems if it's the polystyrene type (the usual cheap one from hardware stores). Some people have switched to acrylic eggcrate (bought online) for use inside the tank. I'm not sure that the algae growth is significant on polystyrene eggcrate, and it probably won't be an issue if it's buried in the sand.