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I am embarking on first attempt at creating a plenum but have experienced great difficulty in obtaining or even attempting to explain what "egg crate" is. Here in the UK it seems to be a totally alien concept. Can anyone help with a more detailed description of what it's normal purpose is or where I can obtain it - the usual DIY shops just stare blankly at me.

"Egg crate" is called grid diffuser in the lighting industry, if I'm not wrong.  It is normally installed for commercial fluorescent fixtures, made of plastic or acrylic and comes in either matt or clear.  The square openings in the grid are normally 1cm by 1cm. You can alternatively use a UGF plate, although some consider it not as good for diffusion.