LLex Crenata in Planter Boxes

A friend wants me to build her two planter boxes out of cedar. They are to be one (1) box 61"L x 18"H x 26.5"W and one (1) box 29"L x 18"H x 26.5"W. Can anyone offer advice on how the bottoms should be constructed for drainage. How many and where drainage holes should be?
Also, any advice on how to plant the llex crenata in the boxes? I am told that I should use lava rocks for drainage. I am guessing (I'm not a gradener) that I would place the lava rocks on the bottom of the boxes and then fill the box with what? The llex crenatas will be 3 to 4 feet high.
Any advice will be greatly appreciated.
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